Automatic Train Control in Rail Rapid Transit (Part 13 Of
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Appendix D GLOSSARY OF TRAIN CONTROL TERMS The language of rail rapid transit and train con- False Restrictive Aspect—the aspect of a sig- trol technology contains many specialized terms nal that conveys an indication more restric- that may be unfamiliar to the general reader. This tive than intended. glossary has been prepared as an aid to understand- ATTENDANT-a transit employee on board a ing the terminology used in the report. It is also con- train in service whose principal duties are to templated that the glossary may be useful as a oversee safety, provide security, and assist in reference for additional reading on the subject of emergency situations (as distinct from a train ATC and transit system engineering. For this operator, motorman, who is responsible for run- reason, the list of terms defined here has been ex- ning the train). panded to include some background items not needed for the immediate purpose of reading this AUDIBLE CAB INDICATOR-an alerting device, report. on a train equipped with cab signals, designed to sound when the cab signal changes and to con- The principal source of the definitions presented tinue sounding until acknowledged. (See here is the Lexicon of Rail Rapid Transit Safety- Acknowledging Device.) Related Terminology, prepared by the Safety Tech- nology Applied to Rapid Transit (START) Commit- AUDIO-FREQUENCY TRACK CIRCUIT—a track tee of the American Public Transit Association, circuit energized by an electrical current alter- January 1975. The START Lexicon, in turn, draws nating in the audio-frequency range extensively on earlier work by the Association of (15,000-20,000 Hz); also called “high frequency” American Railroads and the U.S. Department of or “overlay” track circuit. Transportation. In addition to START, other AUTOMATIC BLOCK SIGNAL SYSTEM—a sources consulted include General Order No. 127 of series of consecutive blocks governed by block the Public Utilities Commission of the State of signals, cab signals, or both, actuated by occupan- California, August 1967, and several technical cy of the track or by certain conditions affecting specifications prepared by WMATA. In all cases, the use of a block; such as an open switch or a car however, the responsibility for interpretation and for the accuracy and completeness of the defini- standing on a turnout and blocking the main track. (See also Block and Manual Block Signal tions offered here rests with the authors of this report. System.) AUTOMATIC CAR IDENTIFICATION—a ACKNOWLEDGING DEVICE—a manual device system that automatically provides positive used by the train operator to forestall automatic recognition and transmission of individual car brake application on a train equipped with numbers as they pass a fixed wayside point. automatic train stop or to silence the sounding of a cab indicator on a train equipped with cab sig- AUTOMATIC TRAIN CONTROL—the method naling. (See Audible Cab Indicator.) (and, by extension, the specific system) for automatically controlling train movement, en- ASPECT—the visual indication presented to an ap- forcing train safety, and directing train opera- proaching train by a wayside signal; also, the dis- tions. ATC includes four major functions: play presented by a cab signal to an operator in Automatic Train Protection (ATP)--assuring the cab. The aspect is said to be “clear” (proceed safe train movement by a combination of at civil speed) or varying degrees of “restrictive.” train detection, separation of trains running False Clear Aspect—the aspect of a signal that on the same track or over interlocked conveys an indication less restrictive than routes, overspeed prevention, and route in- intended. terlocking. 207 Automatic Train Operation (ATO)--controll- rate of the vehicle is less than the braking rate re- ing speed, programed station stopping, door quested by the automatic train control system. operation, performance level modification, BRAKING—the process of retarding or stopping and other functions traditionally assigned train movement by any of various devices: to the train operator and conductor. Dynamic Braking—a system of electrical Automatic Train Supervision (ATS)— braking in which the traction motors are monitoring of system status and directing used as generators and convert the kinetic traffic movement to maintain the schedule energy of the vehicle into electrical energy, or minimize the effect of delays. which is consumed in resistors and, in so Communication (CS)—interchanging infor- doing, exert a retarding force on the vehi- mation (voice, data, or video) between cle. system elements separated by distance. Friction Braking—braking supplied by a AVAILABILITY—the portion of time that a mechanical shoe or pad pressing against the system is operating or ready for operation; wheels or other rotating surface; also called mathematically, the probability that a system or “mechanical braking. ” system element will be operational when re- Regenerative Braking-a form of electrical quired, expressed as the ratio of mean time be- braking in which the current generated by tween failure to the sum of mean time between the traction motor is returned to the trac- failure plus mean time to restore. [A = tion power supply for use in propelling MTBF/(MTBF + MTTR)] (See also Mean Time other trains. (In ordinary dynamic braking Between Failure and Mean Time to Restore.) the generated power is dissipated in resis- BASE PERIOD—the nonrush hour period of week- tors.) day transit system service. (See also Peak There are two methods of controlling Period,) brake application: BERTH—the space assigned for a train of specified Closed-Loop Braking-continuous modula- length when stopped at a station platform or in a tion (by means of feedback) under the terminal zone. (See Terminal Zone.) direction of the automatic train operation BERTHING—the positioning of a train in its system or the human operator. (See Closed- assigned berth. Loop Principle,) BLENDING—the automatic and simultaneous ap- Open-Loop Braking—braking without plication of dynamic and friction braking, where modulation through feedback from the the effort of each is continuously proportioned to ATO system. achieve the required total braking effect. BRAKING EMERGENCY—irrevocable unmodu- BLOCK—a length of track of defined limits, the use lated (open-loop) braking to a stop usually at a of which is governed by block signals, cab sig- higher rate than that obtained with a full service nals, or both. brake application. Absolute Block-a block into which no train is BRAKING, FULL SERVICE-a nonemergency allowed to enter while it is occupied by brake application that obtains the maximum another train. brake rate consistent with the design of the pri- Permissive Block--a block into which a train mary brake system. Full service braking can be is allowed to enter even though occupied by released and reapplied. another train. BRAKING, SERVICE-braking produced by the BLOCK SIGNAL-See Signal. primary train braking system, BRAKE ASSURANCE—the function provided by CAB SIGNAL SYSTEM—a signal system whereby a subsystem within the automatic train operation block condition and the prevailing civil speed system that will cause the emergency brakes of a commands are transmitted and displayed directly vehicle to be applied when the actual braking within the train cab. The cab signal system may 208 be operated in conjunction with a system of fixed slower than the following train. (See also Closing wayside signals or separately. (See also Signal.) up.) CATENARY—the wire or wires above the track CLOSING UP—running a following train to a posi- (including the messenger, supports, and insula- tion that will allow it to couple with a stopped tion) that carry electric energy for the propulsion leading train. of trains. (See also Contact Rail.) COAST—the moving condition of a car or train CENTRAL CONTROL—the place from which where the propulsion is inactive and, usually, a train supervision and direction is accomplished certain minimum braking is applied. (See also for the entire transit system; the train command Freewheeling.) center. CONDUCTOR-an attendant whose main func- CIRCUIT, TRACK-an arrangement of electrical tion is to operate train doors. equipment, including the rails of the track, that CONSIST (noun)—the number, type, and specific forms a continuous electrical path used for the identity of cars that compose a train. purpose of detecting the presence of trains on the rails; the track circuit may also be used to com- CONTACT RAIL-a rail, mounted on insulators municate commands or other information be- alongside the running rails, that provides electric tween the wayside and the train. energy for the propulsion of trains. (Also known as “Third Rail.”) Check-In/Check-Out—a track circuit system that detects the entrance of the front end of CROSSOVER—two turnouts, arranged to form a a train into a block and the departure of the continous passage between two parallel tracks. rear end of a train from a block for the pur- DEADMAN CONTROL-a safety device that re- pose of determining block occupancy. quires continuous pressure or activity to remain Coded Track Circuit—a track circuit in which activated; used to detect the inattention or dis- the feed energy is varied or interrupted ability of a train operator. periodically for the purpose of transmitting DEPARTURE TEST-an operational test made in commands or instructions to the train or a yard or on a transfer track before permitting the operating train detection apparatus. unit to enter revenue service. Fail-Safe Circuit-a circuit designed to princi- DISPATCH—to start a train into revenue service ples which will cause the actuated device to from a terminal zone, transfer track, or desig- assume its most restrictive position (or a nated intermediate point. state generally known to be safe) when any element of the circuit or system fails. DISPATCHER-a person at central control whose Vital Circuit--an electrical circuit that affects function is to dispatch trains, monitor train the safety of train operation.