SECTION 700
TRAIN CONTROL
SECTION INDEX
Train Control Systems 702
Trip Stops 702
Operation 702
Intermittent Inductive Train Control 704
Object of System 704
Penalty for Failure to Acknowledge 704
Transmission of Control 704
Wayside Equipment and Circuits 708
Inductor Mounting and Location 708
Inductor Control Circuit 708
Locomotive Equipment 708
Electro-Pneumatic Valve 709

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Cut-Out Cock . 709
Acknowledging Button 709
Acknowledging Bell or Whistle 710
Reset Button 710
Mechanism 711
Power Supply Unit 711
Miscellaneous Equipment 711
Locomotive Circuits 711
Continuous Inductive Train Control 714
Frequency-Responsive (FR) Speed Governor 715
Operation 715
Two-Speed Train Control 718
Receivers 718
Two-Phase Amplifier 719
Cab Signal and Motion Detector Light 720
Speed Warning Whistle 721
Clear Bell 721
Acknowledging Horn 721
Acknowledging Contactor 721
Governor 721
Electro-Pneumatic Valve 721
D-c/D-c Power Supply 721
Mechanism 721
Operation
- Two-Speed System 721
Overspeed 721
Change of Indication
- Clear to Restrictive 721
Operation in Restrictive Territory 722
Change of Indication
- Restrictive to Clear 722
Locomotive Circuits
- Two-Speed System 722
Clear Territory
- Speed Not Over 70 MPH 722
Clear Territory
- Overspeed 723
Entering Restrictive Territory
- 40 to 70 MPH 723
Restrictive Territory
- Overspeed 724
Change of Indication
- Restrictive to Clear 724
Penalty Brake Application 724
Suppression of Recurrent Acknowledgment 725
Multi-Speed Train Control Systems 725
701

TRAIN CONTROL SYSTEMS
Block signal systems cause the display of signal aspects appropriate to track and traffic conditions. The indication conveyed by the aspect and the procedures to be followed in train operation under the various indications are established by operating rules. In some circumstances it is desirable to expand the basic signal function by addition of a train control system which provides verification of rule compliance.
Train control systems may be divided into two general kinds: intermittent and continuous. Intermittent systems transmit control functions from the wayside to the train only at fixed points along the roadway, usually in the vicinity of the wayside signals. Continuous systems exercise control throughout each block.
Intermittent systems are typically of the train stop type; i.e., unless the requirements of the system are observed the penalty is a train stop. Continuous systems are usually of the speed-control type, in which the system automatically enforces a speed limit consistent with block conditions. However, continuous systems, too, enforce a penalty train stop if specified operating procedures are not fulfilled.
TRIP STOPS
The trip stop in its basic application is an absolute system which automatically stops any train attempting to pass a signal when the signal aspect and operating rules prohibit such movement.
The stop system operates by mechanical engagement between a trip arm on the wayside adjacent to the rail and a train-mounted trip arm which is connected directly or electrically to a valve in the train brake system. When the signal is clear, the wayside arm is lowered so that trains may pass at will. When the signal requires a stop, the wayside trip arm is raised so that it engages the train trip arm and applies the brakes on any train which runs by.
Depending on operating requirements, permissive features may be included. For example, circuitry for “keying by” (a term derived from the use of key-operated switches) can be provided which causes the wayside trip arm to lower when a wayside circuit element, such as a pushbutton, is operated. Or the wayside arm may be automatically lowered at an interlocking when a call-on signal is displayed. Another option is a timing circuit which lowers the wayside arm a specified time after

an approaching train is detected, expiration of the time interval indicating that the speed of the approaching train is sufficiently slow.
The trip stop because of its mechanical nature has certain limitations in application. Severe snow and ice conditions, for example, could interfere with operation of the wayside trip arm. This can be mitigated by the use of heaters. Again, the train trip arm on a moving train may be activated by wayside ice build-up or some other obstruction. The trip stop has thus found its widest application on rapid transit lines, where conditions that might interfere with proper operation are readily controlled.
Operation
Trip stops have a history extending over many decades, and several designs have been used, involving both electric and electro-pneumatic mechanisms. A GRS designed and manufactured unit is shown in Figures 701 and 702.
Principal components of the mechanism are a 117-volt 60-Hz split-phase induction motor with a gear train, phase-shifting capacitors for the motor

Figure 701. Trip stop mechanism.

702



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windings, heavy compression spring, and a circuit controller, all housed in a cast iron case. The output shaft of the mechanism extends through both sides of the case in suitable bearings and accepts either right- or left-hand attachment of an extension rocker shaft which carries the wayside trip arm.
Wayside trip arms are driven up to tripping position by the compression spring and are driven down against the spring by the motor. When drive-

down is complete, the circuit controller operates to insert an auxiliary capacitor and resistor in series with the main motor winding. The reduced input produces stalled torque sufficient to hold the spring compressed without overheating the motor. If energy supply to the motor is interrupted, the spring returns the wayside trip arm to the stop position automatically. A hold-down hook is provided which can be engaged to hold the trip arm depressed if the installation is taken out of service for any reason.

CIRCUIT
CONTROLLER
DRIVING ARM
CIRCUIT
CONTROLLER
DRIVING LINK

Figure 702. Details of trip stop mechanism.

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Wayside trip arms are adjusted so that the arm rises to a point approximately 21/2 inches above the top of the running rail when in stop position, and lowers to approximately 1 inch below the top of the rail when cleared. Operating time is approximately two seconds. A typical installation layout for a trip stop is given in Figure 703.
INTERMITTENT INDUCTIVE TRAIN CONTROL
The GRS intermittent inductive system of train control is a permissive system which allows a train to pass a restrictive signal without penalty provided that the engineman operates an acknowledging button or contactor when passing the wayside element.
Transmission of control between the wayside and the moving locomotive is accomplished without mechanical contact between the wayside element and locomotive element. The status of a relay contact, closed or open, controls the clear or restrictive condition of the wayside element. No energy is required in the control circuit. Transmission of control is effective at all operating speeds up to 100 mph.
Object of System
The object of the intermittent inductive train control system is to enforce observance of restricting indications of wayside signals by requiring the engineman to ‘acknowledge”, i.e., to operate a button or contactor, when passing signals displaying such indications. “Restricting indications” include all indications other than “proceed”.
Penalty for Failure to Acknowledge
The penalty for failure to acknowledge a restricting indication is an automatic full service brake application which cannot be released until one of the following conditions has been met, depending upon the type of installation:
1. The engineman leaves his accustomed operating position and operates a reset button or contactor located at some other point on the locomotive inaccessible while the train is in motion.
2. A predetermined time interval has elapsed after the engineman has operated a reset button or contactor located convenient to his accustomed operating position. Because of the time delay before brake release begins, an enforced stop occurs.

3. Train speed has been reduced to a predetermined low value and the reset button or contactor operated. With the train at a low speed before brake release begins, an enforced stop occurs.
Transmission of Control
Transmission of control between the wayside signal circuits and the locomotive is accomplished through the interaction of two devices, one mounted beside the track, called the inductor, Figure 704, and one carried on the locomotive, called the receiver, Figure 705.
The inductor has a U-shaped core of laminated electrical sheet steel fitted with pole pieces. It is usually equipped with a winding for control purposes, but this may be omitted for special applications as explained later. The inductor is located on the ties parallel with and outside the gauge line, and with its pole faces
21/2 inches above the top of rail.
The receiver is an electromagnet, with a laminated core of inverted U-shape. The electromagnet carries two windings and has pole pieces of the same spacing as those of the inductor. The receiver is mounted on one of the locomotive journal boxes by means of suitable brackets. It is adjusted so that as the locomotive moves along the track the receiver passes vertically above each inductor, with about 1½-inch clearance between their respective pole faces.
The method of transmitting control from the wayside to the moving locomotive is shown in simplified form in Figure 706. The circuits on the locomotive are controlled through normally-energized primary relay Ri; if Ri releases, a penalty full service brake application occurs unless the acknowledging contactor (not shown) is in its actuated position. Ri is held energized through its own front contact and the secondary coil wound on the receiver core structure.
Also wound on the receiver core structure is a primary coil which is continuously energized from the d.c./d.c. converter power supply. The current flowing through the primary coil magnetizes the receiver core structure, but the strength of the magnetic flux is limited when the receiver is not over an inductor by the long air gap between the pole pieces. As the locomotive moves along the track between inductors, the magnetic and electrical conditions in the receiver remain unchanged, and Ri is held energized.

704

During the interval in which the receiver passes over an inductor, conditions affecting the magnetic flux in the receiver change rapidly. At this time the inductor provides a good magnetic path between the pole pieces of the receiver with relatively small air gaps at each pole. As a result, a surge of magnetic flux builds up in the receiver, inducing a voltage in the secondary coil which bucks the voltage from the power supply.

Curve A-B-C-D-E in Figure 707 shows the effect this induced voltage would have on the current passing through Ri if Ri were not a stick relay. Starting with the normal current at point A, the bucking voltage causes the relay current to drop to a minimum at D. Then, as the receiver moves away from the inductor, the current rises again to its normal value. The small fluctuations on the current curve before and after the main

Figure 703. Typical trip stop layout (trip arm raised).

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Receiver on journal box of diesel locomotive.
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PRIMARY
RELAY Ri
(SHOWN IN THE
ACT OF OPENING)
TO
LOCOMOTIVE
BRAKE CONTROL
CIRCUIT

LAMINATED ST R UCT U RE

POWER SUPPLY
I D.C./D.C. LOCOMOTIVE
LNVTR
J PWR. SOURCE
+
RECEIVER
PIECES
INDUCTOR

DIRECTION OF MOVEMENT

RECEIVER
APPROACH ING
INDUCTOR

RECEIVER MOVING OVER IUCTOR

RECEIVER
PAST INDUCTOR

RECEIVER CENTERED
OVER INDUCTOR

current dip are caused by minor flux variations as the receiver approaches and leaves the inductor, and have no effect on the over-all operation of the system.
In actual practice, of course, this complete curve would not occur, because relay Ri would drop as soon as the current became less than the

dropaway current shown in Figure 707 and would then remain down because of the opening of the stick circuit.
Since the system must distinguish between restrictive and non-restrictive conditions, provision is made for controlling the effect of the inductor on the receiver in accordance with signal mdi-

Figure 706. Principle of intermittent inductive control, wayside to train.

NORMAL CURRENT THROUGH RELAY Ri

D
Figure 707. Current through relay Ri.

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cations. This is done by the choke coil (sometimes called “control” coil) wound on the inductor core, as shown in Figure 706. When the signal is restrictive, the signal control relay is down, leaving the choke coil open-circuited. Under this condition no current can flow in the coil, and the inductor produces a flux surge in the receiver to drop Ri, as we have already seen. But if the signal is clear, the signal control relay is up, and the choke-coil circuit is closed through a front contact on the signal control relay. Now when the receiver passes over the inductor, and the flux starts to build up in the receiver-inductor magnetic circuit, the voltage induced in the choke coil causes current to flow through the coil. According to the laws of induced currents, the magnetic flux produced by this current opposes the magnetic flux which causes the current. As a result, the net change in flux is much less than the change which occurs when the choke coil is open-circuited, and the bucking voltage which appears in the secondary coil is much smaller. Curve A-F-C-G-E in Figure 707 shows the current variation in relay Ri under these conditions. The current does not go below the dropaway value at any point, so that Ri remains energized while the receiver passes over the inductor.
To summarize: when the signal is restrictive, the inductor choke-coil circuit is open, and the inductor produces a flux change in the receiver which causes Ri to drop. When the signal is clear, the inductor choke-coil circuit is closed, and the flux change produced in the receiver is not enough to drop Ri.
Note that the transmission of control between inductor and receiver requires no energy in the inductor winding. Whether or not a locomotive receives a train control pulse depends only on the inductor choke-coil circuit. If this circuit is open, primary relay Ri drops when the receiver passes; if the circuit is closed, Ri remains energized. At locations where it may be desirable to require acknowledgment at all times, such as a signal which is not capable of displaying a clear aspect, an unwound inductor can be used. Such an inductor has no choke coil to suppress the flux buildup, and so always produces a train control pulse in passing receivers.
Since aknowledgment proves that the engine- man is alert, the system permits him to retain full control of the brakes when a restrictive inductor is passed while acknowledgment is being made. This is done by circuits through the acknowledging button or contactor which prevent a brake application when Ri drops. To avoid the chance that

the acknowledging button or contactor might be continuously held in the actuated position, making the train control system ineffective, timing functions are included in the associated circuit. These produce a penalty brake application if the button or contactor is held closed for more than 15 sec orids.

Wayside Equipment and Circuits

The basic equipment required on the wayside for the intermittent train control system is the inductor itself, a relay contact which is closed when the signal indicates proceed and is open at all other times, and the necessary wires for connecting the inductor to the relay contact.
In cases where a permanent restriction is required, the only wayside equipment needed is an unwound inductor.
Inductor Mounting and Local ion
The inductor is usually mounted on first-quality ties 9 or 9½ feet long. Mounting arrangements are designed to maintain permanent alignment of the inductor with respect to the running rail.
The inductor is placed sufficiently far in approach of the insulated joints at a signal so that it will act upon a receiver before the leading wheels of the locomotive pass the first insulated joint. This distance depends on the length of locomotives operated, but is generally between 60 and 75 feet.
Inductor Control Circuit
Figure 708 shows a simplified circuit illustrating the principle of inductor control. Note that the inductor control circuit is taken through a front contact of the D relay. This means that the inductor is in its non-restrictive condition only when the 0 relay is picked up, i.e., the signal is green. The engineman must therefore acknowledge all signals other than green to be able to pass the inductor without receiving a penalty train stop.
Locomotive Equipment
In addition to the receiver, a typical locomotive is equipped with the following train control apparatus:
1. Electro-pneumatic valve
2. Cut-out cock
3. Acknowledging button or contactor
4. Acknowledging bell or whistle
5. Reset button or contactor
6. Mechanism
7. Power supply unit
8. Miscellaneous wiring and air piping

708

A typical arrangement of the above, using an acknowledging whistle, is shown in Figure 709.
Electro-Pneumatic Valve
The electro-pneumatic valve, Figure 710, is a solenoid-operated air valve which is normally held energized. This valve is controlled through the train control circuits so that passing a restrictive inductor causes th electro-pneumatic valve to become de-energized unless the acknowledging contactor is actuated at the time. When the electropneumatic valve is de-energized, it opens a control air line which causes the brake application valve (part of the air brake system) to initiate a penalty full service brake application. If such an application occurs, the engineman may, if desired, increase the application to emergency by his brake valve. He cannot, however, obtain release until the electro-pneumatic valve is re-energized.

Cut-Out Cock
The cut-out cock is used to cut the electropneumatic valve out of service in case of trouble in the train control equipment.
Acknou’ledging Button
The acknowledging button, Figure 711, is mounted in the locomotive cab in a position convenient to the engineman. By holding the button closed for a short time as the locomotive passes a restrictive inductor, the engineman averts an automatic brake application.
A time delay relay operates to apply braking if the button remains closed for more than 15 seconds. In older installations, a lever actuated contactor with a self-contained timing-out mechanism was used for acknowledgment.

Figure 708. Typical circuit, inductor control.

ELECTRO PNEUMATIC VALVEPOWER SUPPLY UNIT

RECEIVER MECHANISM CASE

Figure 709. Typical arrangement of intermittent inductive train control apparatus on locomotive.

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Acknowledging Bell or Whistle

An audible signal, which may be a bell or an air whistle, assists the engineman in making acknowledgment. A typical single-stroke gong used for this service is shown in Figure 712. Figure 713 shows a cab arrangement in which a whistle valve is used.
The audible signal sounds momentarily each time a restrictive inductor is passed while acknowledgment is made. Occurrence of the signal informs the engineman that the train control circuits have reacted to the restrictive inductor and that the acknowledging button or contactor may be released. Sounding of the signal also verifies that the train control equipment is responding properly to the restrictive condition.
Reset Button
The reset button, Figure 714, is used for resetting the train control circuits to permit brake release after a penalty automatic brake application has occurred.
A penalty full service automatic brake application indicates a condition of sufficient importance to warrant stopping the train. This may be accomplished by time delay circuits which hold the brakes on for an extended interval after operation of the reset button. Or, if locomotives are equipped with governors, time delay circuits may be omitted and the train stop enforced by preventing brake re Figur

710. Electro-pneumatic valve.

Figure 711. Acknowledging button.

Figure 712. Acknowledging belL

710



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lease until the governor indicates speed reduced to a pre-established low value. In either case the train is stopped before brake release is effective.
In some earlier installations, train stop was enforced through time delay provided directly by a lever-actuated reset contactor which included a

built-in mechanical timer. In other early installations, time delay was not used, but the reset contactor was mounted externally on the locomotive. The mounting location was such that the engine- man could operate the contactor only after the train had stopped and he had alighted from the locomotive.
Mechanism
The mechanism, Figure 715, includes relays and related items associated with the locomotive train control circuits. Test jacks are provided for facilitating required departure and periodic tests. The mechanism is shock-mounted in a protective case, and is installed where space permits on a locomotive.
Pou’er Supply Unit
The power supply unit is a solid-state d.c./d.c. converter which operates from the locomotive control d-c supply and delivers the 32-volt d-c energy required by the train control system. The power supply unit also serves to isolate the train control system from the locomotive wiring.
Miscellaneous Equipment
In addition to the foregoing major items, the locomotive train control equipment includes wiring, conduit and rubber hose (for wires), plug couplers for electrical connections, and piping for air system connections.
Locomotive Circuits
Figure 716 shows in simplified form the basic circuit of a locomotive equipped with intermittent inductive train control for forward running. Ac. knowledging and reset contactors are of the push-

Figure 713. Acknowledging whistle valve, electro-pneumatic valve, and cut.out cock in cab of diesel locomotive.


.1

Figure 714. Reset button.

Figure 715. Locomotive mechanism, intermittent inductive train control.

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BELL
CIRCUIT
WHISTLE
CIRCUIT

V
TO BELL OR WHISTLE CIRCUITS

I

Figure 716. Simplified locomotive circuit intermittent train control, forward running.

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button type, with the reset button installed remote from the engineman’s operating position. Optional circuit arrangements for either an acknowledging bell or whistle are indicated. The circuit is drawn for the normal condition, i.e., as it is with the train proceeding between inductors with the brakes under full control of the engineman.
The circuit functions of the units shown symbolically in Figure 716 are as follows:
ACK. Acknowledging button. When the button is pressed, contacts X and Y close, and Z opens. Opening of contact Z removes energy from the timer relay which, after timing out, releases its contacts to open the EPV circuit and apply the automatic brakes.
AGA. Time delay contactor. Enforces time limit on use of acknowledging button.
BALLAST RESISTOR. A resistor whose resistance increases with increasing current and decreases with decreasing current, thereby tending to maintain current constant and to offset effects of changes in the supply voltage.
BELL. Provides audible acknowledging signal when energized.
CAPACITOR. Shunted around the coil of relay Ri to improve relay response.
CR2. Capacitor for storing energy to ring bell.
EPV. Electro-pneumatic valve. When de-energized, opens an air line which in turn causes the application valve to initiate a penalty full service brake application.
P. Primary winding of the receiver. See Figure 706. PILOT LIGHT. Lights when a train control brake
application occurs. Enables engineman to distinguish such brake applications from those produced by other control devices on the locomotive. (Furnished by railroad or locomotive builder.)
Ri, R2, R3. Plug-in relays which determine the action of the train control circuits.
RESET. Reset button. Provides contacts for reestablishing normal circuit conditions after an automatic brake application.
S. Secondary winding of the receiver. See Figure
706.
WV. Whistle valve. Sounds acknowledging whistle when energized.
100-OHM. Resistor shunted across R2 contact to suppress arcing.
Referring to Figure 716, junction P2 is normally energized from B32 through front contacts of R2

and R3. From junction P2, current flows through the following circuits to C (common):
1. Through normally-closed contact W of AGA, a normally-closed reset contact, and the EPV to energize EPV.
2. Through a front contact of Ri and the coil of R2 to energize R2.
3. Through the coil of R3, the ballast resistor, and P to energize R3 and P.
4. Through a front contact of Ri, S, the coil of Ri, the ballast resistor, and P to energize S and Ri.
When the receiver passes over a restrictive inductor, the bucking voltage induced in S causes Ri to drop, as previously described. When Ri drops, the coil of R2 is de-energized by the opening of the Ri front contact, and R2 drops. This opens the R2 front contact in the circuit between B32 and junction P2, but if the acknowledging button has been properly operated, energy still reaches junction P2 through ACK X, now closed and, when a whistle is used, through WV. As a result, R3 and the EPV remain energized. The current flow through WV causes the acknowledging whistle to sound.
With R2 down and ACK Y closed, a circuit is established for re-energizing Ri, and Ri picks up again, followed by R2. When R2 comes up, WV is shunted out by an R2 front contact, and the whistle is silenced. If an acknowledging bell is used, when R2 picks up it rings the acknowledging bell as the charge on capacitor CR2 is transferred to the bell coil.
At the time the whistle is silenced or the bell sounds, the engineman should release the acknowledging button, since with the button actuated, ACK 2 contact is open, de-energizing timing relay AGA. If he fails to release it within approximately 15 seconds from the start of acknowledgment, the time delay in the timing relay runs out. If this happens the timing relay drops out, and the EPV becomes de-energized, which results in an automatic brake application.
However, since R3 is not de-energized in this case, EPV will be re-energized as soon as the acknowledging button is released. The engineman may then regain control of the brakes after such time penalty as may be provided in the brake system.
If a restrictive inductor is passed without acknowledgment, Ri drops, followed by R2. But now ACK X is open, so that when R2 drops, R3 also drops, opening the stick circuit through the R3 front contact, de-energizing EPV and lighting the pilot light. The resulting brake application cannot

713

be released until the reset button has been operated, for with the R3 stick contact open the only means of re-energizing Ri, R2, and R3 is through the reset contacts.
Operating the reset button closes the contacts which connect B32 directly to R3 and Ri, and opens a contact in the EPV circuit. R3 and Ri come up, with R2 following Ri, re-establishing the relay stick circuits. When the reset button is released, the contact in the EPV circuit closes, EPV is re-energized, and the system is restored to normal condition. Note that the reset contacts when closed energize Ri, R3, and P, but that S is out of the circuit until the reset contactor is released. This prevents S from being affected by the buildup of primary flux.
In installations in which the reset button is mounted within reach of the engineman, a timer in the mechanism case provides a preset time-delay after the release of the reset button before closure of the reset contact in the EPV circuit. When this arrangement is used, the engineman may operate the reset button immediately after a penalty brake application has occurred, re-energizing Ri, R2, and R3 at once. However, since the EPV circuit is

not re-energized until after the time delay has expired, the system still enforces an automatic stop.
Figure 717 is the same circuit as Figure 716, redrawn to show actual terminal and connection arrangements. As already mentioned, this circuit provides protection for forward running only. Forward-running protection requires a single receiver mounted on the right-hand side of the locomotive. When both forward and reverse run- fling are desired, an additional left-hand receiver is required, together with relays for selecting the receiver to be connected into the control circuits.
CONTINUOUS INDUCTIVE TRAIN CONTROL
Intermittent inductive train control requires the engineman to acknowledge restrictive signals but otherwise leaves train operation in his hands, including observance of speed limit rules. It may be desirable, however, to have speed limits automatically enforced. This is accomplished by continuous inductive train control systems.

NOTE
BELL, R12,I CR2 NOT INCLUDED
WHEN WHISTLE VALVE (WV) IS
USED.

Figure 717. Typical locomotive circuit, intermittent inductive train control, forward running.

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A continuous inductive train control system leaves control of the train brakes in the hands of the engineman as long as he operates within speed limits, but makes a full service automatic brake application if overspeed is detected. If a train control brake application occurs, the engineman can obtain any additional braking effect, such as emergency, which the brake system may provide. But he cannot release the brakes until all speed requirements and penalties of the control system have been satisfied. Release of a train control brake application does not occur automatically; the engineman must follow a specified release procedure.
Continuous control requires continuous transfer of information on block conditions between the wayside and the locomotive. This is accomplished by inductive linkage between the track current in the rails and receivers mounted on the locomotive, exactly as with cab signals. Because of this, many of the components used in continuous control systems are the same as those used for cab signals, and a cab signal is included as part of the train control system. We may, if we wish, regard continuous train control as a cab signal system to which has been added speed-detection and brake- applying components, with circuits arranged to apply brakes when speed exceeds the limit permissible under the aspect displayed by the cab signal.
It is not intended that a continuous train control system take over the engineman’s control of the brakes unless necessary. Hence warning devices, such as whistles and horns, are included to inform of an impending train control brake application, so that by taking appropriate manual brake action he may “suppress” it. On the other hand, if he fails to heed the warnings, the train control application will reduce train speed more drastically (possibly down to a complete stop) than would have been required under manual control. This serves both as an incentive to the engineman to observe the requirements of the system, and as a penalty for failure to do so.
Frequency-Responsive (FR) Speed Governor
The function of the speed governor in continuous train control systems is to monitor train speed and to enforce speed reduction if authorized speed is exceeded. The governor also enforces any penalty speed reduction, including complete stop, that may be imposed.

Early designs of governor used a centrifuge mechanically coupled to a locomotive axle. As centrifuge position varied with train speed, a linkage opened and closed contacts used in the control system.
Centrifuge governors have been superseded by electronic designs, typified by the GRS frequency- responsive (FR) speed governor. The FR governor involves two functional elements: (1) a frequency generator which converts train speed into a varying-frequency output; (2) electronic circuits which process the frequency output to determine if speed is within limit. The electronic equipment is in a protected environment remote from the generator, usually in the housing for train control equipment. An underspeed relay (USR) driven by the electronic circuitry is energized when train speed is within limits and releases when speed is excessive. Contacts on USR are used to control system response.
The frequency generator includes a toothed rotor, driven from an axle or other propulsion shaft, which turns in a magnetic field. With the rotor turning, air gaps in the magnetic circuit vary as the rotor teeth change position in the field. Resulting flux changes in a stationary coil induce an output voltage which is fed over a pair of wires to the electronic unit. Frequency of the output is proportional to the rpm of the toothed wheel, hence to train speed.
An outboard frequency generator for use on diesel locomotives mounts on a journal box, Figure
718. This unit, Figure 719, incorporates a small toothed rotor driven from the axle, which turns inside an internally toothed stator to produce the variable-frequency output.
An inboard generator used on a rapid transit car is shown in Figure 720. In this design a toothed rotor clamped to the drive shaft rotates in proximity to a magnetic probe. Although the configuration is different from the outboard generator, the end result is similar: an a-c output with frequency proportional to speed.
Operation
The block diagram, Figure 721, shows the functional relationship of FR governor elements. For purposes of explanation, the signal from frequency generator FG will be ignored for the moment.
The check oscillator produces an output frequency which corresponds to an overspeed output from FG. The check frequency may be varied by the system to reflect different speed limits imposed by signal restriction. With the check oscillator turned on, the overspeed frequency is fed to the amplifier

715

STATOR
ROTOR

Figure 719. Outboard frequency generator, cover off.

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and shaper, which outputs a stream of uniform pulses at the same frequency as the input. The overspeed frequency filter and pulse generator discriminates sharply between underspeed and overspeed inputs. When overspeed input is present, the filter produces a characteristic signal indicating this fact, otherwise no output occurs. The overspeed level to which the filter responds may be varied externally through circuits selected by the train control system.

Overspeed output from the filter and pulse generator, further processed by amplification and shaping, is applied to a voltage-doubler driver. The driver dwells in one circuit state when overspeed exists and reverts to another state when overspeed is absent. The driver must continually switch between these two states to activate the voltage doubler. Doubled voltage (superplus) causes current flow through biased neutral relay USR to the plus side of the system power supply. If short cir

-.
:
-‘

Figure 720. Inboard-mounted frequency generator, rapid transit car, rotor on drive shaft.

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CLOSES LOW SPEED)

cuits or other failures occur, reverse current flow through USR from power plus to power minus will not operate or hold the relay.
Another output of the voltage-doubler driver keys the check oscillator on and off. When the driver reacts to the overspeed check signal, it turns the oscillator off. With overspeed input absent the driver reverts to its opposite state again turning on the oscillator, and the process continues. Continual alternation of the driver produces super- plus voltage to energize USR as previously mentioned.
Existence of superplus voltage demonstrates that the governor circuits are responding properly to the alternate presence and absence of an over- speed input frequency. If input frequency is uninterruptedly underspeed or uninterruptedly overspeed, superplus voltage will not be produced.
Refer now to frequency generator FG. If the train is underspeed, FG output frequency is rejected by the filter, thus circuit operation is as already described. But if train speed is excessive, FG generates a continuous overspeed frequency output. Thus the alternating overspeed-underspeed sequence required to energize USR is not present, and USR releases to initiate the brake response characteristic of the particular control system.
A motion detector circuit verifies that FG is functioning. The motion circuit senses generator output at lowest speed and responds by energizing motion detector relay MDR. Contacts on MDR may be used to control an indicator light, and may also be incorporated in the control system to inhibit

train operation if MDR does not pick up within a preset interval after a train start has been initiated.
Two-Speed Train Control
A two-speed continuous inductive train control system enforces one speed limit in clear territory and a substantially lower limit in restrictive territory. Typical values are 70 mph and 20 mph, and these will be used for purposes of discussion. Since the speed limits are enforced by the speed governor on each locomotive, different limits may be established for different types of service. Thus passenger and freight service locomotives may be held to different limits by appropriate adjustment of the governors on such locomotives. If trains also operate over trackage not equipped for train control, the track-to-train communication elements of the system can be cut out of service while retaining the governor function to enforce a maximum allowable speed in such territory.
To minimize the occurrence of unnecessary penalty train stops, audible and visual cues are provided by the system which enable the engine- man to retain control by responding appropriately. However, if he fails to do so a penalty train stop is imposed.
The components of a typical two-speed system are shown diagrammatically in Figure 722.
Receivers
The principles of inductive transfer of track circuit energy between the running rails and locomotive receivers, and the use of electronic amplifi CHEC

FREO. SELECTION AS REQ BY SYSTEM)

TO
SYSTEM •-

OPENS (OVERSPEEDI

Figure 721. Block diagram, FR governor.

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pict23.jpg

cation to build the weak receiver voltage up to levels suitable for relay operation, are discussed in the section on cab signals. Illustrations of typical receivers are shown in the same section. The receivers and amplifier for two-speed train control, although different in details, operate on the same general principles.
Tu’o-p base Amplifier
In the two-speed system, rate codes are not used. Instead, the clear condition is indicated by the presence of steady a.c., typically 100 Hz, in the rails, and the restrictive condition by the absence of a.c. Since coding is not used, protection is provided against foreign a.c. in the rails by filtering out frequencies other than that used in the control system, and by phase comparison of the output of each receiver coil in a two-phase amplifier.
Figure 723 illustrates diagrammatically the operating principles of the two-phase amplifier.
Inputs from each receiver are filtered and amplified, in parallel but separate channels, to produce 100-Hz Class B push-pull output. With Class B operation each channel outputs only a half wave. This half wave has a fixed phase relationship to the input a.c. from the receiver for that channel.

The push-pull amplifier output is applied to the primary winding of a transformer incorporating a toroidal core of magnetic material which has a rectangular magnetization characteristic. As the half-wave output of Phase A increases, the core shows little change in magnetization until the output reaches a fairly high level, at which time the core changes very rapidly from counterclockwise magnetic saturation to clockwise saturation. The result is a sharp pulse of secondary voltage in the secondary winding which then decays according to the time constant of the rectifier and relay load.
During the succeeding half-cycle Phase A output disappears, and Phase Boutput produces a counterclockwise magnetizing force. Again the core does not respond appreciably until the counterclockwise magnetizing force reaches a critical level, at which time the core changes abruptly from clockwise to counterclockwise saturation. This again produces a secondary voltage pulse, but of the opposite polarity to that occurring for the clockwise transition. The succession of secondary output pulses, converted to d.c. by a bridge rectifier, energizes the primary relay to indicate presence of 100-Hz track current.

CLEAR
BELL

SPEED
WARNING
WHISTLE

ACK.
HO RN

E.P.
VALVE

CAB
SIGNAL
GOV. IND. LIGHT

LOcO.
D.C
SUPPLY

ACK.
OJNT.

Figure 722. Components of two-speed continuous inductive train control.

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pict24.jpg

Note that the relay will not pick up if only one amplifier phase is operating. Since the half-wave output from one phase is always in the same direction, the magnetization of the core remains unchanged, hence no secondary output is produced. Also, unless Phase A and Phase B outputs are essentially 180 degrees out of phase, they will tend to buck each other. Transformer primary current is thus reduced below the critical level necessary to reverse core magnetization.
The condition necessary for opposite phasing of the push-pull outputs is that 100-Hz a.c. must be flowing in opposite directions in each rail at any instant. As Figure 723 shows, when current flow is toward the engine in the right-hand rail, it must be away from the engine in the left-hand rail, and

vice versa. Foreign a.c. flowing in only one rail, or in parallel in both rails, does not meet this requirement, even if it should have frequency components which could pass the input filters. Train control a.c. flows toward the train through one rail, across the wheel/axle circuit of the locomotive, and flows away from the train through the other rail. This meets the phase requirements of the system.
Cab
Signal and Motion Detector Light
The cab signal provides two aspects: green for the clear indication, red-over-yellow for restricting. The motion detector light, which may be included in the cab signal housing, is a white light which lights to indicate that the governor is operating and that the train is in motion.

INPUT FROM
R.H. RCVR.
oOOOOO
INPUT FROM
L.H. RCVR.

XFMR
OUTPUT
TO RECT.

:‘“J

cwf
ccw/ L/

_____ — — —-— -----SATURATED CW

_____ SATURATED CCW

Figure 723. Operating principles, two-phase amplifier.
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TWO-PHASE AMPLIFIER

—J

ØA OUTPUT A TO C.T.
ØB OUTPUT CT TO B
TOROID
FLUX

\J

0



pict25.jpg

pict26.jpg

Speed Warning Whistle

D.c./D.c. Power Supply

The speed warning whistle sounds in clear territory when speed exceeds 70 mph. In restrictive territory, the whistle sounds when speed exceeds 20 mph. The whistle is similar in appearance to the acknowledging whistle used with the intermittent inductive system, Figure 713.
Clear Bell
The clear bell rings momentarily each time block conditions change from restrictive to clear. This attracts the engineman’s attention to the new block conditions so that he may take prompt advantage of the improved situation.
Acknowledging Horn
The acknowledging horn sounds when a train enters restrictive territory (after speed has been sufficiently reduced) and periodically thereafter while the train is proceeding under restrictive conditions. The horn informs the engineman that an operation of the acknowledging contactor is required.
Acknou’l edging Conlactor
The acknowledging contactor must be operated by the engineman each time the acknowledging horn sounds, to prove his alertness to restrictive conditions. Acknowledgment must be made within approximately five seconds of the time the horn sounds, under penalty of a train control full service automatic brake application which will stop the train before it can be released. Proper acknowledgment silences the acknowledging horn and allows the engineman to retain control of the brakes, provided he does not violate the restrictive territory speed limit.
Governor
The frequency-responsive speed governor detects overspeed conditions with respect to the speed limit in effect at any given time as established by the control system. It also provides indication of decreased speed to permit preliminary brake release after a train proceeding at high speed has entered restrictive territory as described later. The motion detector output of the governor controls the motion indicator light, and also controls a circuit which eliminates need for recurrent acknowledgment if a train is standing in restrictive territory with engine brakes applied.
Electro-Pneumatic Valve
The electro-pneumatic valve is of the same general design as described for the intermittent inductive system. When de-energized, it opens an air line which causes the air brake system to initiate a brake application.

The power supply unit is a solid-state d.c. to d.c. converter which operates from the locomotive d-c control voltage supply and delivers the 32-volt d-c energy required by the train control system. The power supply unit also serves to isolate the train control system from the locomotive wiring.
Mechanism
The mechanism consists of: (1) the filters and two-phase amplifier for the receivers; (2) the primary relay; (3) the relays associated with the audible indicator, electro-pneumatic valve, and acknowledgment; (4) a motor-operated timing unit which operates time-delay contacts required by the system. These devices are shock-mounted in a mechanism case, located in any convenient location on the locomotive. Plug-connection is used throughout for ease of maintenance.
Operation - Two-Speed System
For ease of explanation it is convenient to consider separately the operation of the two-speed continuous inductive train control system under various block and speed conditions that may be encountered.
Overspeed
In clear territory (green cab signal aspect), if train speed exceeds 70 mph or in restrictive territory (red over yellow aspect) when train speed exceeds 20 mph, the speed warning whistle sounds, followed in approximately five seconds by a penalty brake application which will stop the train. The engineman may avoid the penalty by suppressing:
i.e., by moving the brake handle to the service position, to initiate a brake application. Suppression must occur within the five-second period between the sounding of the whistle and the initiation of the penalty.
When a full service brake pipe reduction has occurred, the brake handle may be returned to the lap position. The speed warning whistle continues to sound until train speed is reduced to the limit consistent with the signal aspect. When the whistle turns off, brakes may be released.
Change of Indication - Clear to Restrictive
Assuming train speed above 40 mph when the indication changes from clear to restrictive, the speed warning whistle sounds, and the engineman must suppress within five seconds to avoid a penalty brake application. When speed decreases to 40 mph the whistle turns off, and the acknowledging horn sounds, requiring operation of the acknowl 721

edgment contactor. After acknowledgment is made, brakes may be released without penalty. However, speed must be further reduced to 20 mph within 75 seconds subsequent to first acknowledgment or a penalty brake application will occur unless brakes are manually reapplied.
Operation in Restrictive Territory
While proceeding in restrictive territory, the engineman is required to acknowledge every 120 seconds after the first acknowledgment, as indicated by the acknowledging horn. Failure to acknowledge within five seconds after the horn sounds results in a penalty brake application.
When the train is standing in restrictive territory with a pressure of 35 pounds per square inch in the locomotive brake cylinders, recurrent acknowledgment is not required. However, the engineman must acknowledge prior to proceeding.
Change of Indication - Restrictive to Clear
A change of indication from restrictive to clear causes the clear bell to sound, calling the engine- man’s attention to the change. No specific action is required. Any brake application initiated to comply with the restrictive condition may be immediately released.

Locomotive Circuits - Two-Speed System
Figure 724 shows the circuit for the two-speed continuous inductive train control system described above. The circuit has been simplified from the circuit used in an actual installation by converting double-break contacts to single-break and by omission of various details such as provision for system cutout, but basic operating principles are retained. The diagram shows circuit conditions on a train proceeding at 70 mph or less in clear territory with brakes released.
In clear territory, 100-Hz energy is in the rails, hence primary relay PR is up, energized by the two- phase amplifier. Motion detector relay VD, down only when the train is stationary, is also up.
The speed limits under various conditions are 3, 20, 40, or 70 mph, depending on which of the speed limit inputs to the FR governor is selected by the system.
Clear Territory - Speed Not Over 70 MPH
As the train proceeds, electro-pneumatic valve EPV is energized, closing the associated air line and leaving brake control with the engineman. With
CLOSED WITH FULL SERVICE BRAKE PIPE REDUCTION.
© CLOSED WITH 35 LBS IN ENGINE BRAKE CYLINDERS.
13 CLOSED WHEN TIMING MOTOR RUNS.
(13 CONT A OPENS 75 SEC AFTER ACKNOWLEDGMENT.
B AND C CLOSE. U OPENS WHEN ACKNOWLEDS. OCCURS BANDCOPEN, SCLOSES 120 SEC AFTER ACKNOWLEDS.
® CLOSED WHEN ELECTRO-PNEUMATIC VALVE IS CLOSED. © CLOSED WITH BRAKE HANDLE IN SERVICE POSITION
OPEN IN LAP AND RELEASE POSITIONS

2W AWP. OUTPUT (+1
20 AMP OUTPUT I—I
SPEED WARN. WHISTLE

II
6 RECUR. ACK
SUPPRESS SWITCH
Figure 724. Simplified circuit, two-speed continuous inductive train control.
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pict27.jpg

EPV closed, the suppression check switch, pressure actuated on the high-pressure side of the pipe to EPV, is also closed, energizing check relay
SCK.
With PR and VD up, the green cab signal aspect is lighted through PR 32/33, and the white motion detector indicator is lighted through VD 22/23.
The acknowledging contactor ACK is in normal released position and acknowledgment repeater relay ACP is down. Primary repeater relay PRP is up, energized through PR 12/13 and ACP 11/12.
With PR up, the 70-mph speed limit is selected through SCK 12/13 and PR 35/36. Since train speed does not exceed 70 mph, underspeed relay USR is also up, energized by the FR governor. Govvernor repeater relay GP is therefore up, energized through USR 25/26 and PR 16/15.
When it is up, slow-acting relay SA energizes EPV through its 25/26 contact. Capacitance shunted across the SA coil is adjusted to obtain release time of slightly more than five seconds. With acknowledging relay AR (described later) down at this time, SA is energized via PRP 35/36, GP 35/36, AR 34/35, and AR 32/31. If this circuit is opened, SA releases between five and six seconds later, deenergizing EPV unless alternative energy circuits to EPV have been closed.
Under the conditions specified (clear territory, speed not over 70 mph, brakes released) the timing motor in recurrent acknowledgment contactor RAC is not running, and acknowledgment relay AR is de-energized. The brake handle is not in service position, hence the immediate suppression switch is open. The suppression switch is also open, since it closes only when a full service brake pressure reduction has been made. With EPV energized, the suppression check switch is closed. Neither of the audible indicator (acknowledging horn, clear bell) is energized, hence are silent.
Clear Territory - Overspeed
If train speed exceeds 70 mph in clear territory, USR releases, and GP, which is slightly slow release, follows after a short delay. Release of GP sounds the speed warning whistle, with energy fed through PRP 35/36, GP 35/34, and SCK 17/18. With GP 35/36 open SA is de-energized. SA releases five to six seconds thereafter, opening the energy circuit to EPV through SA 25/26.
EPV will release, producing a penalty brake application when SA drops unless the engineman, alerted by the whistle, has suppressed within the release time of SA by moving the brake handle to the service position. This action closes the imme diat

suppression switch, maintaining energy on EPV via the suppression check switch.
When speed drops to 70 mph, USR picks up, followed by GP, turning off the whistle. With GP up, SA is re-energized and picks up to re-establish the normal energy path to EPV. Brakes may thus be released when the whistle silences.
Entering Restrictive Territory - 40 to 70MPH
When a train enters restrictive territory, PR releases. The green cab signal aspect goes dark, and the red/yellow aspect lights, energized through PR 32/31. With PR down a GP stick circuit is set up through contact A of recurrent acknowledging contactor RAC, GP 15/16, and PR 14/15. Closing of PR 14/15 in the GP stick circuit opens PR 16/15 through which GP has been energized, but GP is sufficiently slow release to bridge the PR transfer.
Release of PR drops the speed limit to 40 mph, selected through SCK 12/13, PR 35/34, and GP 12/13. Since speed exceeds 40 mph, USR releases. With PR down, PRP is de-energized and releases approximately two seconds after PR, energizing the upper winding of the whistle valve, through PRP 35/34 and SCK 17/18, sounding the whistle. PRP 35/36 also opens, de-energizing SA, which releases in about five seconds. To hold EPV energized, it is necessary to suppress by moving the brake handle to service position. When a full service reduction is reached, the pressure actuated suppression switch closes, and the brake handle may be moved to lap position. With brakes on, speed reduces. When speed has decreased to 40 mph, USR picks up. This silences the speed warning whistle by applying energy, through USR 22/3 and PRP 24/23, to the lower winding of the whistle valve. This current flows in the direction which bucks out the effect of the energy still being applied to the upper winding. The circuit through USR 22/23 and PRP 24/23 in addition supplies energy (1) to the heel of acknowledging contactor ACK, and (2) through AR 15/14, to the acknowledging horn, causing it to sound.
Sounding of the horn informs the engineman that acknowledgment is reqtiired. When ACK is actuated, ACP picks up. ACP sticks up on energy fed through USR 22/23, PRP 24/23, RAG contact D, and ACP 16/15.
ACP 33/34 is now closed, energizing the timing motor in RAG. When the motor rotates, centrifugal contact 35/36 closes, proving the motor is operating and that timing requirements will be enforced. Acknowledging relay AR now picks up, energized through PRP 35/34, RAG 35/36, and ACP 35/36, silencing the horn by opening AR 15/14. The en-

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gineman releases the acknowledging contactor when the horn turns off, but ACP remains up through its stick circuit.
Rotation of the timing motor drives two cams in RAC. Shortly after timing starts, the 120-second cam closes RAC contacts C and B, and opens D. With RAG 41/42 closed, the motor now continues to run until the 120-second cam has completed a revolution. With RAG 43/44 closed, AR sticks up through AR 12/13.
When RAC 46/45 opens, the stick circuit through AGP 15/16 is opened, de-energizing ACP. Because AGP is slow release, the pickup circuit for AR through ACP 35/36 and the motor energy circuit through ACP 33/34 do not open until after RAC 41/42 in the motor feed, and RAG 43/44 in the AR stick circuit, are closed. The motor thus continues to run and AR holds up after ACP releases.
When the acknowledging contactor is released, ACK normal contacts close, re-energizing SA by energy feeding from AGK heel contact through 36/35 of AR, now stuck up, SA coil, AR 32/33, and PRP 31/32. SA comes up to energize EPV, allowing brake release to be initiated. Train speed will continue to decrease below 40 mph, however, because of time required by the brake system to respond. Speed will normally drop to 20 mph or less in an interval of approximately 60 seconds following acknowledgment.
Seventy-five seconds after the timing motor has been energized, the cam follower on the 75- second cam moves off the lobe, opening the stick circuit holding GP. GP releases and transfers the speed limit selection from 40 mph to 20 mph through GP 12/11. Assuming normal conditions, train speed by this time is within limit and no action is necessary.
Should speed exceed 20 mph, USR will release. Opening of USR 22/23 removes energy from the lower coil of the whistle valve and from SA. The whistle sounds, and the brake handle must be moved to service position to suppress release of EPV. When speed drops to 20 mph, USR comes up, the whistle silences, and brakes may be released.
The RAG cam follower on the 120-second cam moves off the lobe 120 seconds after the first acknowledgement. This stops the motor by opening RAG 41/42 and opens the stick circuit on AR through RAG 43/44. AR releases, de-energizing SA by opening AR 33/32 and 36/35. Simultaneously, the acknowledging horn sounds, energized through USR 22/23, PRP 24/23 and AR 15/14.
Acknowledgement within the prescribed 5 seconds silences the horn and resticks AR through

the previously described sequence of AGP pickup, AR pickup, plus RAG startup and run.
While operating in restrictive territory, RAC will time out every 120 seconds, indicated by sounding of the horn. Acknowledgment within the five- second limit must be made each time. Since the energy supply circuit to SA under these conditions is through the normally closed contact of the acknowledging contactor, it is necessary that the contactor be released promptly upon silencing of the horn to avoid release of SA.
Restrictive Territory - Overspeed
If train speed in restrictive territory exceeds 20 mph, USR releases, opening the energy supply to SA through USR 22/23, PRP 24/23, AGK normal and AR 36/35. With lower coil energy off, the whistle sounds. Moving the brake handle to service position within five seconds holds EPV. When speed drops to 20 mph, USR picks up, the whistle is silenced, and brakes may be released.
Change of Indication - Restrictive to Clear
When the locomotive enters clear territory, PR picks up. This extinguishes the red-over-yellow aspect by opening PR 32/31 and lights the green aspect through PR 32/33. At the same time, the speed limit goes to 70 mph, selected through SGK 12/13 and PR 35/36. PRP is down when PR is down. When PR picks up, PRP is energized through PR 12/13 and ACP 11/12. However, PRP is slow acting and does not pick up immediately, so that for a brief period PR is up while PRP is still down. This energizes the single-stroke clear bell through PR 32/33 and PRP 22/21, informing the engineman of the changed condition. When PRP picks up, energy is removed from the bell by opening of
PRP 22/21.
As previously described, SA is held energized in restrictive territory through front contacts of AR, which is held up through a stick circuit. When PR, PRP, and GP pick up, the stick on AR is broken and AR releases. But at the same time the circuit conditions previously described for a train proceeding at not over 70 mph in clear territory are re-established. Since release time for SA is five seconds, it remains up during the contact transitions.
No action is required of the engineman in response to a change from restrictive to clear.
Penalty Brake Application
If either suppression or acknowlegement is not performed as required, SA releases, de-energizing EPV, which opens the brake pipeline. Should this

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happen, the suppression check switch sensing loss of pressure also opens. This releases SCK, dropping the speed limit to 3 mph, selected through SCK 12/11. With the suppression check switch open, EPV can be re-energized only through SA, possible only when USR is picked up. Brake release therefore cannot be started until train speed has dropped to 3 mph, which assures a train stop before brakes are actually off. Should a penalty stop occur in clear territory, as from overspeed without suppression, brakes also are held on until speed is reduced to 3 mph, after which they may be released. In restrictive territory, speed must be reduced to 3 mph and the acknowledging contactor also operated before brakes can be released.
Suppression of Recurrent Acknowledgement
Provision is included for avoiding the need for recurrent acknowledgement if a train is standing still in restrictive territory as at a station, or on track such as in yards not equipped for 100-Hz control energy. With the locomotive stationary, motion detector relay VD is de-energized, and VD 12/11 is closed. With 35 pounds of pressure in the locomotive brake cylinders the recurrent acknowledgement suppression switch will also be closed. A circuit is thus established which will hold the AR stick circuit, described earlier, through PRP 35/34, the closed recurrent acknowledgement suppression switch, and VD 11/12. Thus, afterthefirstacknowledgement, AR will remain up and no further acknowledgement will be required while conditions remain unchanged. When locomotive brakes are released prior to moving, AR will drop, the acknowledging horn will sound, and acknowledge-

ment must be made to avoid a penalty train brake application. Once motion has started, VD picks up and recurrent acknowlegement is required.
Multi-Speed Train Control Systems
We have seen how using inductive transfer of information by track energy “on” or off”, a two- speed train control system operates. We also know (see section on Cab Signals) that we can increase the information available to the locomotive by coding the track energy. Thus train control systems are available which enforce not just two, but a series of speed limits. For example, we can if we wish enforce four different speed limits in conformance with the clear, approach medium, approach, and restricting aspects of the associated cab signal.
This is accomplished by selecting the speed limit input to the FR governor through relay contacts controlled by the onboard decoder units as they respond to the various code rates. If a train runs faster than the speed limit permitted for the corresponding indication, an audible warning is sounded, and brakes must be applied manually to avoid a penalty stop. Typically, a second audible indicator, such as a bell, is included to alert the engineman to improved conditions.
Where coded track energy is used, it is not necessary that the elctronic amplifiers be of the two-phase type, since the coding itself protects against foreign current.

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