Toronto rapid transit signals

The Toronto rapid transit system signals are a system of light-based indicators to give instructions to the Toronto Transit Commission's (TTC) trains on the Toronto rapid transit system. 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.

Overview

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.

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, then 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 signalling 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.

Block signals

Block signals are the most commonly used signals on the Toronto subway and RT. They are used to keep trains properly spaced, and are controlled by the trains themselves, based on their distance relative to other trains. The following block signals are used by the TTC.

A series of block signals south of Yorkdale station.

25px	Proceed
25px	Proceed with Caution, next signal is currently red
25px	Stop. Passing this signal trips the train stop.
25px	Entering Timed Block, next signal is red only due to grade timing
25px	Timed Block, timer has not yet run out (red light flashes when timer is about to run out), next block is timed as well as lunar aspect is indicated (in this example this signal would only clear to yellow)

GT (grade timing) is used in sections where a sharp turn requires a speed limit or where a downhill section would cause a train to accelerate to an unsafe speed if the driver were unwary. When entering a block which is subject to GT, one of two things controls the signal: the distance to the train ahead, or grade timing. If the current state of the signal is due to proximity to the train ahead, the white light below the signal (termed "lunar aspect" by the TTC) will not be illuminated. The lunar aspect is used only to indicate that the signal is being controlled by GT.

As well, despite the images shown above, a flashing red light may be shown without the lunar aspect. The flashing red indicates the end of a GT block whose timer has not expired, while the lunar aspect indicates the start of a GT block whose signal is currently being controlled by GT. Therefore, the signal at the end of the last block of a GT section may be flashing red to indicate that the timer has not yet run out, but that location will never have a lunar aspect since the next block is not subject to GT.

Interlocking signals

File:TTC interlocking signal.JPG

A TTC interlocking signal

Interlocking signals are used in interlockings, which are any areas where train movements may conflict with each other. They are controlled by either human operators or a computer, not by the trains. Interlocking signals also tell operators which way points are set. The following interlocking signals are used on the TTC.

25px	Proceed, Points Set to Straight
25px	Proceed With Caution, Points Set to Straight, Next signal is currently red
25px	Proceed With Caution, Points Set to Diverge
25px	Stop And Stay
25px	Call On (Train has been given permission to pass red signal)
25px	Entering Timed Block, Points Set to Straight, next signal is red only due to grade timing
25px	Entering Timed Block, Points Set to Diverge
25px	Timed Block, timer has not yet run out (top red light flashes when timer is about to run out), next block is timed as well as lunar aspect is indicated (in this example this signal would only clear to yellow over green)

Signal numbers

All signals have an alpha-numeric number that relates to their location within the subway system. The number is assigned using the Chain system of measurement, whereby a signal's number is assigned based on the nearest chain measure.

Each line or portion of a line has an assigned letter, and that precedes the number ascertained by the Chain measure. Signals that are on a northbound portion of track use the nearest even valued chain measure, where signals on a southbound portion of track use the nearest odd valued chain measure.

Line	Signal prefix	Even	Odd	Chain 0 mark
Yonge	N (northbound only) S (southbound only)	northbound	southbound	Does not exist (continues from the University numbers)
University	U	northbound	southbound	South of St George station (counts up towards Museum)
Spadina	SP	northbound	southbound	North of St George station (counts up towards Spadina station (YUS))
Bloor-Danforth	B	westbound	eastbound	West of Kipling station (counts up towards Islington)
Sheppard	SHP	westbound	eastbound	West of Sheppard station (counts up towards Bayview)

Temporary signals

In work zones, staff place yellow beacons on the track bed between the rails to inform train operators that a 'slow order' is in effect; the first beacon is usually accompanied with a speed restriction sign indicating the speed limit for the affected area. A green beacon indicates the end of a work zone and allows operators to resume normal operation. In outdoor sections, yellow and green flags are also used for the same purpose. A flashing blue light at track level indicates workers may be present, subway operators are required to sound their horn, and follow the signals of track workers when approaching and passing them.

See also

• 1995 Russell Hill subway accident
• Railway signalling

Videos

• Toronto Subway (TTC) Signal System Video

External links

• SIGNALS: Questions and Answers about the Signals in use by the Toronto Transit Commission (PDF Format)