Toronto Subway and RT - Signals

Signals

The TTC, like most transit systems, uses a system of light-based signals to give instructions to their trains. They use block signals commonly, as well as interlocking signals. The system has been relatively unchanged since it was first installed on the Yonge Line in the 1950s.

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 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 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 signaling 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 just "T" in white.

Station timing, a method of evening out trains, has been imposed on certain stations with interlocking (or home) signals. These signals turn to a red aspect as a train passes it, and is forced red for a variable amount of time. This time depends on the distance between the last train that passed the signal, and the train that comes after the next train. This system is computerized, and can accurately calculate the relative distances. If the next train is closer to the train before than the train after, the signal will hold the train at the station. If the next train is closer to the train after it than the train before it, then the signal will clear.

There are several limitations to this signaling system that can result in "signal problems" and "signal delays". One of the most common problems is track down. A track down occurs when a block gets a false reading and places signals into the danger position even when there is no train occupying the block. This can occur if debris interrupts the block by grounding out the track circuit mimicking the electric circuit caused by an actual train in the area.

When a signal fails to clear, depending on the area, there are three different ways to rectify the situation. On home signals, and station timed signals transit control can perform a "call-on" where an orange aspect blinks and the trip arm is released even when the aspect displayed is red. The second option is a "key-by". Some signals have a plunger that the operator can stop, reach out the window, operate the plunger dropping the trip arm and then operate the train to a less restrictive signal. Where neither of these options exist, the only way to get past a defective signal is to "trip through". The operator at slow speed must trip the signal (which in turn trips the train and places it into emergency). The crew must then reset the emergency valve (by going out the front door of the train) before proceeding.

Since 2008, the TTC is planning on upgrading their signal system within the next 15 years as they prepare to switch to automatic train operation.

The Scarborough RT uses Automatic Train Control (ATC) to control vehicle movement and therefore does not have traditional wayside signals. In the event that the computer that controls the trains fails to operate normally in-cab signaling can be used as a backup. This allows the operator to take control of the vehicle and operate it based on information displayed from within the cab of the vehicle. In the event of a total computer failure it is possible to operate the vehicles using the absolute block technique (verification by a controller or supervisory personnel that train movement is safe to the next point and at a speed where the train will be able to stop safely using line of sight observation). This option is usually exercised only to get the train to the next station to offload passengers when it is expected that the delay could be lengthy. The SRT also differs from the subway in that it uses axle block counters to confirm train location instead of a track circuit.