Subway Signals: A Complete Guide
All divisions of the New York City subway use what is now called "wayside color-light block signalling", that is, signals are physically located to the side of the track (as opposed to being overhead, or transmitted directly to the train), utilize the colors of lights (as opposed to semaphores or other devices, or positions or shapes of lights), and rely on dividing tracks into discrete blocks (or track sections or track circuits) to electrically detect the presence of trains. Nearly all of the routes of the current subway are signalled with classic wayside block signalling, whose principles have not changed markedly since the opening of the IRT in 1904. (In-cab signalling and recent position- and radio-based technologies are slowly being deployed on some lines; the first to be cut over is the "L", chosen because it shares no trackage with other routes, and has a relatively simple service pattern, i.e. no express service.)
Tracks are divided into track circuits (or sections, or blocks), normally hundreds of feet long, by insulated joints in the rails. Trains' wheels and axles short-circuit the two rails to indicate their presence in a block. While every signal occurs at an insulated joint (a block boundary), not every joint has a signal at it. Each block can either be occupied (any portion of a train in any part of it) or vacant (no train anywhere) at any time. Every signal has a control length, defined as the distance, a number of whole blocks beyond the signal (i.e., in the direction of traffic governed by it), whose occupancy forces the signal to red ("stop") - the control length of a signal enforces that separation between trains. A signal being clear (green or yellow) attests that there are no trains, or parts of trains, or other obstructions anywhere in its control length, that is, that its control length is clear of dangers. The New York City subway relies on automatic train stops to "trip" noncompliant trains, that is, force them to obey signals that indicate "stop". Control lengths and signal placements are chosen such that an out-of-control train "tripped" by a train stop will come to a halt before reaching the obstruction or danger. Control lengths also overlap each other to keep trains at least one signal apart.
Signals in the vicinity of switches ("switch-tracks", "points" (UK)) must be and are controlled in a coordinated fashion known as interlocking, which usually involves a human tower operator selecting the routings of trains.
The pages listed below explain more about the various kinds of signals and signal features, and include illustrations of the signals' aspects (distinctive displays of colored lights) and the corresponding indications (what they mean). The discussions are oriented towards those who have spent some time looking at the system from the front of a train, and have already seen, if not fully understood, the signal system's appearances and behaviors. They should be browsed as a set.
Approach, Automatic, and Marker Signals
The most common signals, plus train order signals and miscellaneous light signals.
The mechanism that forcibly stops trains that disregard signal indications.
Grade time and station time signals that enforce train speed limits.
How signals and switches are controlled together.
The signals with two or more "heads" near switches.
Fixed metal signs used in signalling.
Holdout Signals and Bidirectional Traffic
Signals that enforce the direction of traffic on a length of track.
Single-Line Signal Diagrams
How to understand the "single line" diagram format used to express the layout and capability of signals.
NXSYS, Signalling and Interlocking Simulator
A complete interactive simulation of virtually all features of New York City Subway signaling.
Descriptions and graphics in this section (unless otherwise noted) are Copyright © 1997-2002 Bernard S. Greenberg (contact).