Introduction 602
Operating Advantages 602
Application of Cab Signals 603
Control of Cab Signals 603
Wayside Circuits and Equipment 603
Locomotive Equipment and Circuits 605
Organization of Locomotive System 605
Power Supply 607
Receivers 607
Receiver Operation 607
Filter 608
Amplifier 608
Whistle 609
Acknowledging Contactor 609
Cab Signal 609
Locomotive Circuits 610
Signal Control Circuits 610
Acknowledging Circuits 610
Train Control and Automatic Train Operation 613


To simplify discussion, this section treats cab signals as an independent function. It should be remembered, however, that cab signals are typically used in conjunction with wayside signals, or are incorporated as elements of a train control system.
Cab signals represented the earliest application of electronics to railroad signal technology. The development of vacuum tubes in the first decades of the twentieth century made it possible to amplify weak signals to a level adequate to operate vital relays. An experimental cab signal installation was placed in service in 1923, and by 1925 permanent installations were in operation. These systems represented the first application of electronics to a service outside the communications industry. Figure 601 shows how the cab indicator of a system using vacuum tubes was adapted to the windshield centerpost of a Diesel locomotive of the same era.

Progress in the intervening years has reflected the advance of electronic technology. Modern cab signal systems employ soild-state components exclusively. In many cases the system includes high-frequency track circuits for which the track energy is also generated by solid-state electronics.
Detailed description of cab signal equipment would involve examination of electronic circuitry outside the scope of this publication. A basic understanding can be obtained, however, by means of block diagrams which show general function and relationship of system elements.
Automatic block systems using wayside signals have been developed to a high degree of perfection, and the signal indications given by such systems can be acted upon with confidence. But on occasion, through no fault of the wayside signal,

Figure 601. Cab signal aspects are always visible, regardless of terrain or weather.



the engineman cannot see the signal aspect, possibly on account of fog, smoke, or snow. In such cases the engineman must consider the signal as displaying its most restrictive aspect and must control his train accordingly. If this occurs, and the signal is actually not restrictive, time is lost unnecessarily.
Even when visibility is good, if a train enters a block for which the wayside signal is restrictive, the engineman must reduce speed to the maximum permitted for that indication and continue through the entire block at no greater speed. But suppose the condition responsible for the restriction changes immediately after the train enters the block so that it would be safe to resume normal speed. The engineman, even if he sees the less restrictive wayside signal ahead, cannot take advantage of the improved situation until he actually reaches the signal since the wayside signal does not apply to the block he is in at the time. He would again lose time by proceeding through the block at a speed lower than necessary.
GRS cab signals eliminate such situations by displaying in the locomotive cab the signal aspect applicable to block and track conditions immediately ahead of the locomotive. Cab signals give a continuous, up-to-the-second indication which is always visible, and which enables the engineman to proceed at maximum permissible speed even when outside visibility is limited.
Since the cab signal changes its indication at the same time block conditions change, he is able to increase speed promptly when an indication becomes more favorable instead of proceeding to the next wayside signal at unnecessarily low speed. Because of these operating advantages, cab signals help to increase average speeds with resultant improvement in track capacity and on-time perfo rmance.
To enhance the effectiveness of the visual cab signal, an audible warning is provided which sounds in the cab each time the signal indication becomes more restrictive and which continues to sound until silenced by operation of an acknowledging contacto r.
Cab signals are adaptable to all types of motive power, and can provide the conventional signal aspects. Figure 602 shows the aspects and indications recommended by the MR.

Cab signals may be used with wayside signals, or they may be used without wayside signals in conjunction with speed control. When used with wayside signals, the cab signal aspects (for a threeindication system) for each block in the rear of a train are shown in Figure 603a. If used without wayside signals, an arrangement like that shown in Figure 603b may be used. Note that in the latter the red-over-yellow aspect is carried back a block to the rear of the block occupied by the leading train, to provide the most restrictive cab signal indication before a following train actually enters an occupied block. When wayside signals are used with cab signals, this is not necessary, since the wayside signal shows the entering point of the occupied block.
The information needed for control of the cab signal is transferred from the track to the locomotive by inductive coupling between the track and circuits on the locomotive. Since inductive coupling is effective only when an a-c component is present in the track, steady-energy d-c track circuits cannot be used alone.
One means of obtaining a varying current is to use a-c track circuits, or to superimpose an a-c component on the d-c used for wayside signal control. A system in which ac. is continuously on the track for a clear block, but is not present for a restrictive block, provides two-indication cab signals.
If we wish to have more than two indications, we can code the a-c track current at various rates, such as 75, 120, and 180 times a minute, just as is done with coded track circuits for control of wayside signals. The locomotive is then provided with decoding equipment similar to that used on the wayside, with the cab signal controlled accordingly, to provide three, four, or more indications.
Fortwo-indication uncoded cab signals, it is necessary to apply a.c. to the track when the clear signal is desired in the cab, and to remove a.c. at other times. This a.c. may be superimposed on d.c. used for wayside signal control. For multiple-indication coded cab signals, the wayside circuits apply the



RULE 281.)


RULE 282.)


RULE 285.)


RULE 290.)






Figure 602. Cab signal aspects and indications recommended by the AAR.




a. Three-indication cab signal with wayside signals.



b. Three-indication cab signal, no wayside signals.

Figure 603. Signal aspect diagram.

proper code to the track to obtain the desired indication. Again, the code pulses most have an a-c component to provide inductive coupling.
Wayside circuits and equipment suitable for track coding are described in sections of this publication dealing with coded track circuits.
Cab signal equipment installed aboard a locomotive or rail rapid transit car operates under very severe conditions. Equipment is subjected to vibration and shock and may be exposed to weather or to the possibility of mechanical impact. Rugged design, careful shock mounting, and appropriate protective housings are essential. It is also necessary to adapt systems to special operating requ irements of different users, and to “package” equipment to fit the onboard space available, which may be very limited.
Figure 604 shows relays and electronic units on an equipment rack installed on locomotives which operate over a number of different railroads. This system has operational flexibility to respond to a variety of track installations and to produce the signal aspects specified for the various roads.
Figures 605A and 605B show another arrangement, for equipment used for a cab signal system on trains in commuter service. Space considerations made it desirable to package the electronic and relay units on separate mounts to fit specified locations.
Relays and electronic units used in cab signal systems are plug-connected for easy maintenance, minimizing the risk of tying up motive power if trouble develops in the system.

Figure 604. Onboard relay and electronic equipment package, capable of operating in various modes on any of five different railroads. All units are plug-in.
Organization of Locomotive System
The organization of a typical three-indication cab signal system is shown in Figure 606. For purposes of explanation, it will be assumed that the system is designed for operation with coded a-c (100-Hz) track energy in which 180 code rate is used to obtain the clear indication and 75 code rate to obtain the approach indication. Absence of code, resulting from a train shunt or any other reason, produces the restricting indication.
The code being fed to the track rails is picked up inductively by two receiver äoils, one of which is mounted on each side of the locomotive (or head- end transit car) in front of the leading wheels, directly over the rails, about six inches above rail head. The energy picked up by the receiver coils passes through the filter where frequencies other than 100 Hz are filtered out. The filter output is coupled to a solid-state amplifier which amplifies and processes the low-level coded input to produce two outputs similar to those developed by the combination of master decoding transformer and TR relay described for coded track circuits. One of



B. Relay equipment packaged for under-seat installation.
Figure 605. Relay and electronic equipment for installation on commuter service car. All units are plug-in. Note shock mounts between inner racks and outer support structure.

A. Electronic equipment packaged for car locker installation.




Figure 606. Organization of three-indication cab signal system.





these outputs is d.c., regardless of code rate; the other is a coded output applied to a 180-rate decoder unit. When 180-rate code is received, d-c outputs are produced by both the decoder unit and the amplifier. With 75-rate only, the amplifier d-c output is present, but not the decoder output. With no code, neither output is present. Relays driven by these outputs control circuits to give the clear (green) indication for 180-rate code, and the approach (yellow) indication for 75-rate code. With no code received both relays are released, and the restricting (red-over-yellow) indication results.
Associated with the signal control relays are acknowledging relays which cause a whistle to sound each time an aspect becomes more restrictive. The whistle is silenced by operation of the acknowledging contactor, an act which proves the engineer is aware of the warning.
Power Supply
The power supply, in addition to converting the locomotive supply to the required voltage level (typically 28 volts d.c.), isolates the signal system from the general locomotive wiring. This eliminates degradation of the signal system that might result from electrical faults elsewhere on the locomotive.
The receivers, Figure 607, used with low-frequency (typically 100 Hz) track energy consist of multi-turn coils on laminated iron cores. The coils are encapsulated for maximum protection in their exposed position, Figure 608. Wire leads from the coils are brought out through a flexible cable or rubber hose.

Receiver Operation
The operating principle of the receiver is shown in Figure 609. In this illustration, the rail is energized with coded a.c. from the track transformer. The rail, like any conductor carrying a current, is surrounded by a magnetic flux, indicated by the circles around the rail. The direction of the flux depends upon the direction of the current in the rail. When a.c. flows in the rail, the flux alternates in direction.
The receiver coil and core are within the magnetic field of the rail current, hence are magnetically excited. When a.c. is in the rail, the excitation alternates, and an alternating voltage is induced in the receiver windings. If the a.c. in the rail is coded, i.e., is turned on and off at some code rate, say 180 times a minute, then the a-c voltage in the receiver winding goes on and off at the same rate.
If the other rail of the track were indicated, the instantaneous a-c current and flux directions would be opposite to Figure 609, and the instantaneous output polarity of a receiver over that rail would be reversed. The two receivers on a locomotive are series-connected with reversed phasing so that their outputs are additive. If stray a-c currents occur, as from 60-Hz power systems, they tend to flow in the same direction in both rails. Phase reversal of the receivers acts to cancel out any effect such currents might have. (For noncoded two-speed train control systems, the receivers are independently connected to a twophase amplifier for reasons discussed in the section on train control).

Figure 607. Receiver



Figure 608. Receiver coil (indicated by arrow) mounted on locomotive.
The filter unit is designed to pass a relatively narrow frequency band bracketing the track a-c frequency. It serves to keep interfering a-c frequencies which might be picked up by the receivers from reaching the amplifier.
The amplifier is a solid-state electronic unit whose functional organization is shown in Figure 610. Low-level signal from the filter is amplified and demodulated to produce d-c code. This is further

amplified and applied through a driver stage to an electronic pole changer. Output of the pole changer is fed to one or more decoders (depending on the number of codes employed) which produce d.c. to operate associated relays when their codes are present.

Figure 609. Principle of inductive coupling between rails and locomotive receiver coils.




Figure 610. Functional organization of amplifier, with 100-Hz 180-code input.





In addition, the amplifier includes a voltage booster stage, similar to a voltage doubler, which provides a d-c output identified as superplus” substantially above the positive d-c voltage supplied to the amplifier. This output is produced when any of the code rates is present, but not otherwise. Superplus energy is returned through biased neutral relay L to the positive terminal of the power supply. With code present, L is picked up. With no code, it is released. Since L is biased opposite to the current direction produced by the power supply, it will not pick up if a wiring or circuit fault develops which causes supply current to flow through the relay from B28 to C28.
Cab signal systems use an air whistle to alert the engineman to more restrictive signal aspects. The air supply is obtained from the locomotive air system and is blocked from entering the whistle by a normally-energized solenoid valve. Signal change to a more restrictive aspect de-energizes the valve, permitting air flow to sound the whistle. The alarm continues until the engineman operates the acknowledging contactor to reset the valve.
The use of an air whistle with closed-circuit valve control permits a warning to be produced in the event of electrical power failure, or a disruption of wiring to the valve.
Acknowledging Contactor
The acknowledging contactor is mounted so that it can be operated conveniently by the engine- man. Each time the signal becomes more restrictive the contactor must be operated and releaed to silence the whistle. The circuits are arranged so that the whistle will sound continuously if the contactor is blocked in the depressed position.
Cab Signal
The cab signal unit, Figure 611, is made up of a housing divided into a number of internal compartments depending on the signal aspects required. In front is a colored roundel (or appropriate mask, for position light aspects). In each compartment is a lamp, lighted as required to produce the desired signal aspect.
In some cases, it has been necessary for locomotives to operate over a number of railroads using cab signal systems which differ from road to road. These locomotives have been equipped with a GRS cab signal system which uses the cab sig Wna shown in Figure 612. This signal unit, used with
the equipment rack shown in Figure 604, is arrang e to provide the signal displays required by the
Figure 611. Cab signal indicator for mainline locomo- different railroads, such as color-light or position tives light, by means of a selector knob on the signal



unit. At the same time the display is selected, contacts are actuated which adapt the associated equipment to the cab signal track energy used on the various railroads and to the outputs necessary for the aspects to be displayed.
Locomotive Circuits
Cab signal circuits used on locomotives differ from railroad to railroad but have basic functions in common. These are illustrated in Figure 613, which shows in simplified form a typical locomotive circuit for three-indication signals. The signal path from the receivers through the filter and amplifier has already been traced. When 180-rate code is received, relays 180R and L are picked up. With 75-rate code, 180R is released and L is up. With no code, both 180R and Lare down.
Signal Control Circuits
That portion of the locomotive circuitry used for controlling signal aspects, Figure 614, is derived from Figure 613 by eliminating circuit elements not directly related to signal display. With 180R up, the green aspect is lighted through a front contact on 180R and a back contact on acknow ledgin

relay SP. With 180R released and L still up, corresponding to 75-rate code, the yellow aspect is lighted through a back contact of 180R, front contact of L, and a back contact of SP. With both 180R and L down, red-over-yellow is energized through back contacts of 180R and L.
In addition, when the red-over-yellow aspect is acknowledged, SP picks up and sticks up through a back contact of L (not shown in Figure 614). The signal lamp is then also energized through a front contact of SP. SP is made slow release by the capacitor shunted around its coils, so the signal cannot change from red-over-yellow to a more favorable aspect until approximately three seconds after the stick circuit through L has been broken. This delays the occurrence of a less restrictive indication until a definite code has been established.
Acknowledging Circuits
The portion of the locomotive circuits used for acknowledging is shown in Figure 615. These circuits have been simplified by omtting the duplicate contacts (shown in Figure 613) used to provide double-break control. When the signal is

Figure 612. Convertible cab signal. Provides aspects used on 5 different railroads by turning knob on top.





clear, whistle valve WV is energized through a front contact of 180R, and through back contacts of LP and SP. If the signal changes from clear to approach, 180R releases, de-energizing WV, causing the whistle to sound. Operation of the acknowledging contactor energizes LP through a back contact of 180R and a normally-open contact of the acknowledging contactor. When LP comes up, it sticks up on the circuit through 180R back contact,

L front contact, and LP front contact. Release of the acknowledging contactor then re-energizes WV through 180R back contact, L front contact, two LP front contacts, SP back contact, and the normally-closed acknowledging contact.
If the signal changes from approach to restricting, L releases, opening the stick circuit on LP so that LP also releases. WV is de-energized, and the whistle sounds. Acknowledgment again


Figure 613. Simplified typical locomotive circuit, three-indication cab signal system. (Relays shown in “clear” position).



180R L
11 11

Figure 614. Signal control, simplified circuit.

Figure 615. Acknowledgment operation, simplified circuit.



picks up LP, as in the previous case. In addition, SP also picks up, since it is energized through 1 80R back contact, the normally-open acknowledgirig contact, and L back contact. SP sticks up through back contact 180R, its own front contact, and back contact of L. With SP up, LP is held up through 180R back contact and SP front contact. In addition, WV is re-energized through SP front contact when the acknowledging contactor is released.
When the signal changes from restricting to approach, L picks up and opens the stick circuit on SP. But SP is slow release, so that SP remains up for a period of about three seconds after L picks up. When SP releases, LP remains up because the stick circuit through L front contact has been reestablished. The whistle gives a momentary “peep” when SP crosses over from its front to back contact, but no acknowledgment is required since WV is energized when SP makes its back contact. If the signal now clears, 180R picks up, and LP drops to re-establish the circuit conditions indicated in Figure 615. As before, a momentary ‘peep” of the whistle occurs, but acknowledgment is not needed.

It is apparent that cab signal systems embody a communications channel via the rails which provides information on track and block occupancy conditions directly on board the moving train. By combining speed detection and brake applying apparatus with the cab signal system, we can automatically regulate train speed so that it does not exceed limits appropriate to the signal indication. We can also enforce penalty stops if the engine- man fails to perform certain actions, such as operating the acknowledging contactor or failing to apply brakes when required. Conversely, we can utilize the wayside-to-train communication link to command a train to increase its speed should operating conditions make it advisable to do so. Systems which provide these features are described elsewhere in this publication in discussions of automatic train control and automatic train operation.