Train stop

Part of a railway signalling system, a train stop or trip stop (sometimes called a "tripper") is a train protection device that automatically stops a train if it attempts to pass a signal when the signal aspect and operating rules prohibit such movement, or if it attempts to pass at an excessive speed.

Basic operation

The train stop system consists of two basic components: the trip stop mechanism, mounted on the ground adjacent to the rail, which basically consists of a spring-loaded trip arm and an electric motor; and the train-mounted trip cock that is connected directly or electrically to the train's braking system.

The trip arm is up whenever the train should be automatically stopped. When it is safe for the train to proceed, the trip arm is lowered. This is a vital fail-safe system. The motor drives the trip arm down only when the signalling system determines it is safe for the train to proceed; the spring ensures that the trip arm is raised in all other situations "(stop)". If a train attempts to pass the signal with the trip arm in the raised position, the trip arm makes mechanical contact with the trip cock on the train, causing the train's brakes to be automatically applied and the train comes to a stop.

Wayside trip arms are adjusted so that the trip arm rises to a point approximately 2½ inches (about 6 centimetres) above the top of the running rail when in the stop position, and lowers to approximately 1 inch (2.5 centimetres) below the top of the running rail when clear. Operating time is approximately two seconds.

First installations

The first installations of train stops occurred around 1900 on the underground sections of the tube railways in London and New York.

Types of train stops

There are three types of train stops:
*Trip stop - stops train trying to pass red signal
*Timed train stop - stops train moving too fast
*Fixed train stop - prevents any trains for passing a point

Trip stops

The trip arm is up whenever the signal is not displaying a proceed aspect. If the train tries to pass the signal, the train stop remains raised and trips the train to a stop. When the signal indicates it is safe to proceed "(clear)", the trip arm is lowered, and the train is able to proceed without further hindrance.

Timed train stops

With a timed train stop, the train stop stays raised until the approaching train has shunted a track circuit on the approach for a period of time corresponding to a set speed. If the train approaches at a speed higher than the set speed, the train stop remains raised and trips the train to a stop. If the train approaches at a speed equal or lower than the set speed, the train stop lowers before the train arrives, and the train is able to proceed without further hindrance.

Some timed train stops require the driver to acknowledge a stimulus before being lowered on a yellow signal.

Fixed train stops

Fixed train stops are a type of stop that is immovable. They are positioned before a dead end track to stop a train before it reaches the end. They may also be used at the end of tracks where normal service trains should not pass.

Where used

Because of its mechanical nature, the train stop has certain limitations in application. Severe snow and ice conditions, for example, could interfere with operation of the wayside trip arm. Its widest application, therefore, is on underground rapid transit lines, where conditions that might interfere with proper operation are readily controlled. The New York City Subway (among the first users of train stops) and the Toronto Transit Commission Subway are two such systems where train stops are used. Train stops are also used on some London Underground lines, but are gradually being phased out in favour of ATO and distance-to-go signalling.

References

* "Elements of Railway Signaling" General Railway Signal (June 1979)

See also

* Automatic train protection (ATP)
* Automatic Warning System
* Cab signalling
* Catch points
* Dead-man's control
* Indusi or PZB - German ATP
* Moorgate control
* Railway signal
* Train Protection & Warning System
* Vigilance control