Introduction 1202
Rapid Transit Automation 1202
Automatic Train Control for Rapid Transit 1202
Train/Wayside Communication 1204
Automatic Train Protection 1204
Automatic Train Operation 1205
Station Stops 1206
Door Control 1207
Automatic Train Supervision 1207
Central Control 1208



The principles of signaling used in rail rapid transit systems, particularly as they relate to safety of train movement, are similar to those of mainline signal systems. Track circuits are used for train detection, block systems assure train separation with adequate stopping distance, and interlockings protect against conflicting routes and improper switch operation.
Much of the equipment, such as relays, is also identical with equipment used for mainline operations. Other devices, for example wayside signals, differ in some details but provide the same functions. Design differences are in large measure related to restricted clearances and other special conditions encountered in subway tunnels and on elevated structures.
But rapid transit service does introduce some considerations not met with in mainline operation. An example is safe control of car doors. It is essential that car doors open only when a train is stopped in the proper position at a station, and that doors open only on the side of a car adjacent to the platform. (Provision for emergency door opening is also necessary, but is outside our area of interest.)
Older rapid transit systems typically employ wayside signals, often in conjunction with mechanical trip stops. Low-frequency a-c track circuits, with impedance bonds, are used for train detection. Details on a-c track circuits and trip stops are given elsewhere in this publication. Trains operate under manual control, in conformance with signal indications and rules, backed up by the trip stops. Station stopping and platform positioning are accomplished by the motorman. Door opening is manually controlled, subject to interlocks of propulsion and brake systems, and does not relate to the signal system.
Manual systems have been in use for many decades with very satisfactory results. Their performance in terms of service to passengers depends in great measure on the skill level of operating personnel.
Even before World War II, a GRS system which provided continuous inductive control of maximum train speed was in service on a transit system in the San Francisco Bay area. To meet the need of more recent transit systems for high passenger throughput combined with maximum passenger satisfaction, GRS subsequently developed automated control of rapid transit. The resulting sys tem

provide not only safety assurance, they also include functions related to passenger comfort, passenger information, and management of transit operations.
The level of automation appropriate to different rapid transit systems varies. Optimum cost/benefit ratio depends on local circumstances and is determined by the authority responsible for providing the service. With its long experience and technical competence, GRS can provide useful input to such determinations.
The discussion which follows describes the system of rapid transit automation furnished by GRS for service on the Washington, D.C. Metro, Figure 1201. The features of this system provide a comprehensive overview of the subject. Applications elsewhere, as already stated, would be subject to modification to meet local requirements.
The number of subsystems used by Metro and their extensive interrelationship preclude detailed description at the component and circuit level. Our approach thus will be in terms of function and general principles of operation. Note, however, that many of the elements mentioned, such as high- frequency track circuits with WEE-Z bonds, continuous inductive speed control by means of coded energy in the rails, and the frequency-responsive speed governor are described in detail elsewhere in this publication.
By common usage, the designation Automatic Train Control as applied to rapid transit systems has been broadened to include functions not found in mainline train control systems. Thus, the Washington Metro automatic train control (ATC) system provides not only assurance that trains operate in conformance with signal indications, but also provides fully automated train operation plus additional functions. Figure 1202 indicates the wide range of functions included.
Although trains normally operate in fully automatic mode, each train has an attendant at the head end, Figure 1203. The attendant monitors automatic operation and is available for passenger assistance should circumstances require. A console provides readouts for the attendant’s information, and has provision for manual inputs.
The ATC system includes three primary subsystems:
Automatic Train Protection (ATP)
Automatic Train Operation (ATO)
Automatic Train Supervision (ATS)



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Figure 1201. Passenger station, Washington, D.C. Metro rapid transit system.


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Figure 1202. Block diagram, Washington Metro Automatic Train Control System.




Figure 1203. Train attendant monitors train. Operation is normally completely automatic in passenger-service areas.
The subsystems are coordinated through a dual (active and backup) computer installation at Central Control to achieve an integrated real-time control system.
The ATP subsystem maintains safe stopping distance between trains, ensures safe door operation, and provides control of interlockings. The ATO subsystem starts trains out of stations, regulates train speed between stations, and brings trains to a properly positioned stop at stations. The ATS subsystem automatically selects routes through switching locations, dispatches trains automatically, and furnishes the means to make trains responsive to supervisory commands from Central Control.
Central Control monitors the total transit system and initiates correction commands to smooth traffic flow. Correction commands may be generated automatically by computer, but if conditions warrant, the computer alerts a Central Control operator and informs him of conditions. Corrective action is then initiated by the operator based on computer information output.
The ATC system includes three additional subsystems: Data Transmission, to transfer commands and data between wayside locations and Central Control; Cable Transmission, a system-wide network which provides voice-band circuits for Data Transmission and other Metro communication functions; Automatic Car Identification, which

optically scans coded markers on each car to identify the car numbers of cars in Metro trains and thus provides data for computer processing of car statistics.
Train/Wayside Communication
An important element in the Metro installation is the GRS Metro-LinkTM system for two-way train! wayside communication (TWC). Although nominally a part of the ATS system, Metro-Link has important functions in automatic train protection and operation. It will be helpful, therefore, to discuss this communication system briefly before consideration of the ATP and ATO systems.
Information carried via TWC takes the form of digital messages (i.e., a code formed of “ones” and “zeros”), transmitted through the rails, using a carrier frequency higher than the channel frequencies used for track circuits and speed commands. At the wayside, the WEE-Z bonds used for track circuit and speed command functions serve also as coupling transformers for TWC. The rail- to-train up-link is via separate coils assembled in the same receiver structure with the coils used for reception of speed codes. Train-to-rail down-link is via a 3-foot by 4-foot horizontal loop encased in a heavy polyvinyl chloride housing, Figure 1204, mounted under the train at the head end.
Basic TWC communication points are at stations. However, at certain wayside locations, such as approaches to interlockings or terminals, “flyby” communication is provided. This permits the train to identify itself for automatic route selection through the interlocking, or to receive speed modification messages related to optimizing system performance.
The following information is handled by the TWC system:
Train to Wayside
Train destination
Train number
Train length
Train motion
Train berthed
Station check
Doors closed
Train ready
Automatic Train Protection
The ATP subsystem enforces safe operation. It detects the location of each train and imposes speed limits to ensure safe train separation and

Wayside to Train
Train destination
Train number
ATS acceleration limit
ATS operating speed limit
Station check



to restrict speed where required by curves and grades. Detection is by means of high-frequency track circuits supplemented by single-rail a-c track circuits at locations not suitable for high-frequency circuits, such as crossovers and some areas in interlockings. Wire loops mounted between the rails provide coupling of speed commands to train receivers at such locations.
When a train is detected by its track shunt, a maximum speed command is transmitted to it over the rails (or via inductive loops where a-c track circuits are used) utilizing high-frequency audio channels different from the detection track-circuit frequencies. Coupling of high-frequency energy from wayside equipment to the rails, for both track circuits and command circuits, is through the same WEE-Z bonds. Coupling of commands from the rails to the train is via receiver coils mounted on the front end of the lead car. Receivers are functionally equivalent to those used for mainline cab signaling. However, ferrite cores are used instead of steel, and smaller coils appropriate to the frequencies are employed. The receivers are thus lighter and more compact than those required at lower frequencies. An FR speed governor en-

forces compliance with ATP speed limits establisted by the speed control commands.
To provide the command capacity required, commands are transmitted by rate coding one of two distinct carrier frequencies. The carborne equipment decodes the command by identifying:
(1) the carrier frequency and (2) the code rate. Two carrier frequencies and six code rates provide 12 commands, 10 for speed control and one each for left- and right-side door control.
Automatic Train Operatkn
The ATO system handles start-up and acceleration to running speed, maintains en route speed, and at passenger stations stops the train smoothly at the proper platform position.
The en route speed for the train is determined by one of three sources. First is the ATP speed limit, which indicates the appropriate speed in the absence of modification by the ATS system. When in this mode, train running speed is maintained just below the permitted maximum. A second

Figure 1204. Train-carried transmitting loop for coupling TWC train output messages to rails.



source which can set operating speed is the ATS system, which may instruct the train via TWC to run at a speed appreciably under the ATP limit. For example, a train may be directed to slow down to permit traffic ahead to clear, thus avoiding an unnecessary full stop. The ATS system can also instruct the train to modify its acceleration rate as an additional adjustment of performance. Thirdly, train speed is controlled on a programmed basis for making station stops, as discussed subsequently. Under all conditions, however, the ATP speed limit may not be exceeded.
Speed information is displayed on the train attendant’s console. As indicated by Figure 1205, this includes the ATP maximum speed limit in effect, the operating speed currently called for by ATS, and the actual train speed at the moment.
Smooth operation is maintained by providing advance information to the train on changing track grades, thus enabling the on-board system to anticipate propulsion requirements. Grade information is obtained from inductive wayside markers similar to those used in the station stop sequence discussed later.
ATO also handles routine train dispatching at passenger stations. Train dwell time at stations is controlled by local timers which start trains a preset time interval after arrival. This is normally independent of Central Control. At certain stations, however, dispatching equipment ensures that trains do not leave before a scheduled clock time. Provision is made for schedule variations related

to day of week and holidays. Modifications can also be made locally or from Central Control. Trains can be added to or deleted from the schedule, or held at the platform with doors open or closed.
Station Stops
For a station stop, an ATO speed profile generator on the train takes over. This causes the train to follow a pre-established curve of decreasing speed terminating at zero speed at a target stopping point. The target may be one of several platform positions. Long trains always stop centered at the platform. Short trains may be instructed to berth at center platform, or off-center toward either end of the platform.
The station-stop sequence is initiated and modified by wayside markers, Figure 1206, installed between the rails at approach points 2,700 feet (outer marker), 1,200 feet (middle marker), 484 feet (inner marker), and 160 feet (station platform marker) from the platform center line. The markers employ inert tuned coils that inductively couple momentarily to a train-carried marker receiver coil as the train coil passes over the marker coil. Each marker coil provides a tuned feedback link for a marker sensing circuit on the train. The sensing circuit on the train oscillates briefly at the resonant frequency of the track coil during the interval in which the car coil is within coupling range of the track coil. The occurrence of this oscil Figur

1205. Maximum speed limit, regulated speed for optimum transit system operation, and actual train speed are displayed on train console.

Figure 1206. Marker coils between rails interact with train coil to provide information on grade and distance to go fo1 station stop.




lation, and the frequency at which it occurs, informs the train system that a station is being approached, distance to go, track grade, and desired platform position.
Distance-to-go and grade are constant, and are transmitted by fixed-tuned markers. Marker coils governing platform position have selectable resonant frequencies corresponding to different positions. An additional frequency is available to instruct the train to skip the stop, i.e. to run through the station without stopping. Coil tuning is handled at the wayside by selection circuits which respond to commands from the station attendant’s control panel or from Central Control.
The outer, middle, and inner markers use a pair of closely spaced coils; the platform marker uses one coil. (Platform markers are visible near the trains shown in Figure 1201). The first coil encountered in each pair, and the platform coil, have fixed frequencies which indicate distance. The second coil at the outer marker has a fixed frequency to indicate grade. The second coils at the middle and inner markers are selectably tuned, to provide information on platform position or skip-stop.
The station-stop sequence is triggered at the outer marker. The speed profile generator on the train receives distance and grade information and produces an output to control the speed regulation circuits. A preset nominal 2 mph per second braking rate is modified to compensate for grade. Subsequent on-board adjustments are made automatically as the remaining markers are encountered, to offset any deviations detected between the train’s position as computed on board and as verified at the known marker distances.
A recorded station announcement, via the public address system on board the train, is triggered at the inner marker to provide passenger information on the station stop. A departure announcement is also made in conjunction with the door closing sequence.
Door Control
Door control is an ATP function but is included at this point because of its logical relation to the station stop operation. ATP door control ensures that car doors will open automatically only when the train is stopped at a station with all opening doors aligned with the station platform.
When train speed drops below a preset low value as the train comes into the station, the train transmits a message via the TWC system. The wayside reacts by removing ATP speed commands from the two blocks adjacent to the platform. After the speed commands are removed, an open door command, in the form of one of the two ATP command

frequencies, is transmitted from the WEE-Z bonds at each end of the platform.
If the train is completely within platform limits, the command signal is picked up by the receiver coils at both ends of the train. If the train is not within platform limits, one or more sets of train wheels overlap one or the other of the WEE-Z bonds. This prevents the output of that bond from reaching the receiver. Unless the command is detected at both ends of the train, doors will not open.
In addition to the platform position check, the door control system verifies that there is no output from the motion detector, no speed commands present, power off, and brakes on. There must also be continual repeat-back interchange of dynamically varying check information between the train and the wayside via TWC. Only when all conditions are met are doors permitted to open. The side on which doors open is determined by the frequency used for the open door command.
Automatic Train Supervision
The ATS subsystem controls the arrival and departure of trains from all stations, first by automatic equipment on the wayside and secondly by Central Control computer programs automatically called into play for minor schedule adjustments. The ATS system also provides input for updating the video status displays at Central Control, and supplies data for Central Control computer programs which compile operating statistics on each Metro car.
Communication between Central Control and each train is by means of the Metro-Link system. Train number and destination data are provided by the Central Control computer and stored in the train’s data memory. The train number is reported to Central Control each time the train makes a station stop. The computer follows the train’s progress by block occupancy reports. When the train identifies itself by number at a passenger station, the computer correlates this information with block occupancy and thus is able to associate each occupancy anywhere along the system with a specifically identified train.
Destination data stored aboard the train is transmitted by the train via Metro-Link to obtain automatic routing through interlockings. Destination data is also used by the train for automatic control of destination signs displayed by trains for passenger information.
When a train reports itself at a station, the computer checks arrival against schedule. If variation exceeds a preset tolerance, the computer


decides which corrective strategy to put into effect, or whether the situation requires the attention of the Central Control operator.
ATS adjustment of performance level to keep trains on schedule is handled by the computer by noting the actual time of arrival at a station. The computer then calculates the running time to the next station necessary to meet the current schedule, and transmits to the train the acceleration rate and running speed needed to achieve this result. The train proceeds to the next station on this basis, subject to the priority of ATP speed commands.
The ATS system provides, in addition, two passenger-related functions. It controls platform signs which show train destinations, and it controls surface-embedded lights on the edges of platforms. The edge lights are turned on to indicate the platform at which a train will arrive, and the boarding zone along the platform.
Central Control is the nerve center of the ATC system. Its primary functions are:
1. To monitor performance of the total transit system and to display system status to the central operator. In addition to ATC functions, provision is made for monitoring and status display of the electrification system and of miscellaneous passenger station functions.
2. To select and exercise the control strategies necessary to regulate traffic flow to even out trains.
Central Control capabilities include both automatic control and interactive control in which the central operator initiates action based on displayed information.
The Central Control facility includes a dual computer system with its software; and a control console with video display units. Provision is made for fall-back power supply sources so that operations can be maintained should normal power supply fail.
The two computers at Central Control are large-capacity high-speed units. In the normal operating mode, one computer, designated the control computer, with its associated software, processes input data from the wayside and passenger station equipment, drives displays and alarms, and provides all control action assigned to the computer except rescheduling functions.

The second computer, designated as the backup/Operations Reporting System (ORS) computer, provides rescheduling capability. It also provides ORS capability (general data reporting and processing for the management of Metro) when rescheduling is not being handled. In the event of failure of the control computer, the backup/ORS computer assumes all control functions.
Data on critical system parameters is continually transferred from the control computer to the backup/ORS computer to ensure that an updated base is always available should it be necessary for the backup/ORS computer to assume control.
The control and display subsystem, Figure 1207, permits the Central Control operator to monitor the operation of the system and to initiate corrective action when required. A panel on his control console includes alphanumeric pushbuttons as well as pushbuttons for frequently used functions, such as interlocking controls. Through this control panel the operator also has access to previously designed operational plans and strategies for normal and emergency conditions. These plans and strategies may include automatic processing of the commands necessary to execute them. The operator may, for certain strategies, cause the computer to calculate and display the likely effect of a strategy so that he can then judge whether or not to execute it.
The primary display for the operator is a bank of video display units. There is an overview of each transit route in the form of a schematic track representation. Each train in the system is displayed on the schematic at its actual location. The representation of each train’s position is a train symbol, which denotes the direction of travel and the train’s destination. The train’s run number can be displayed at the operator’s command.
If a train exceeds preset schedule tolerances, or if the computer calculates that the train will exceed schedule tolerances despite the effect of the computer’s automatic control, a special alarm is displayed for that train. Video display units assigned to alarms list any current out-of-tolerance status and equipment failures for the ATC, traction power, and passenger station support facilities.
If the operator needs to manipulate an interlocking, he can call up a detailed display of the interlocking on a separate display. The positions of track switches and other conditions at the location, are displayed in schematic form. With this information available, he can effectively perform the desired operations.


Figure 1207. Operations supervisor at Central Control is informed of progress of each train and other system conditions by a bank of video display units.