The New York City subway primarily uses "wayside color-light block signalling." This system features signals located beside the tracks, employing colored lights to indicate conditions. Tracks are divided into discrete blocks, typically hundreds of feet long, using insulated joints. A train's wheels and axles short-circuit the rails to signal its presence within a block.

Each signal has a "control length," which is the distance of blocks beyond it that must be clear for the signal to show green or yellow. This ensures separation between trains. Automatic train stops are employed to forcibly halt trains that disregard "stop" signals, designed to bring them to a halt before an obstruction.

Signals near switches are coordinated through "interlocking," often managed by a human operator. The system has evolved with the introduction of in-cab signalling and newer technologies on select lines, such as the L train. The article details various signal types, including approach, automatic, marker, time, home, holdout, and bidirectional signals, along with train stops and interlocking mechanisms. Illustrations of signal aspects and their meanings are provided for those familiar with observing the system from the front of a train. The simulator NXSYS allows for interactive exploration of these features.

Key components include insulated joints, automatic train stops, and the L train's adoption of new technology. The principles of this signalling system have remained largely consistent since the IRT opened in 1904.