Toronto subway and RT signals

Toronto subway signals are a system of light-based indicators to give instructions to trains on the Yonge–University, Bloor–Danforth and Sheppard lines of the Toronto subway system, operated by the Toronto Transit Commission (TTC). It uses block signals and interlocking signals. The TTC uses the NX/UR system of signalling, which is also used in New York on the New York City Subway, in Chicago on the 'L', and in Boston on the MBTA.

Line 3 Scarborough has automatic train control and does not use track-side signals.

The system works on fixed signal blocks (a section of track that can be occupied by a train), with lit aspects indicating whether it is safe for a train to proceed into the next fixed block. Interlocking signals or protected signals are used where track features such as crossovers and pocket tracks exist where it is possible to route trains in either direction. The signals are directly connected to a trip arm that has the ability to stop a train if it violates a signal (runs a red light). This safety method is identical to that of the New York City Subway system.

If a train is occupying a block, the next two to four signals behind the train will be red with the trip arms in the danger position so that a train cannot proceed into the area. This allows for a safe stopping distance, even if a train behind violates a signal (the trip arm would trip the train's emergency brakes).

Grade timing is a method of speed control that is worked into the signalling system. In a grade timed section the signal preceding the timed block has a lunar white aspect below the coloured signal. The following signal is red (only because the section is timed) and the signal will blink the red aspect (or the top red aspect in a home or interlocking signal) for a predetermined time before the signal clears. In addition to lunar white signals, grade timed sections are sometimes indicated by a sign with the letters "GT", or simply "T", in white.

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Wikipedia