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Part of a railway signalling system, a train stop, trip stop or tripcock (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 (in some applications) if it attempts to pass at an excessive speed.
The train stop system comprises two basic components. One is the trip arm mechanism, mounted on the ground adjacent to the rail, which essentially consists of a spring-loaded arm connected to an electric motor (or pneumatic cylinder in electro-pneumatic systems). The other is the train-mounted trip cock, which is connected either directly or electrically to the train's braking system.
The trip arm is raised automatically whenever a train should be brought to a halt. When the signalling system determines it is safe for the train to proceed, the motor drives the trip arm down to the lowered position. The spring ensures that the trip arm is raised in all other situations, which is an essential fail-safe provision in case of driver blackouts. 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, thereby bringing the train to a halt.
Wayside trip arms are adjusted so that they rise to a point approximately 2½ inches (about 6 centimetres) above the top of the running rail when in the stop position, and lower to approximately 1 inch (2.5 centimetres) below the top of the running rail when clear. The time taken for the arm to rise or be lowered is approximately two seconds.
There are three types of train stops:
The trip arm is raised whenever the signal is not displaying a proceed aspect. If a train tries to pass the signal, the trip cock on the train strikes the raised trip arm and the train is brought to a halt. When the signal indicates it is safe to proceed (clear or caution), the trip arm is lowered, and a train is able to proceed without further hindrance. In some cases, the trip arm will not be lowered when the signal to which it applies is exhibiting a proceed indication, e.g. when subsidiary signals are cleared, forcing a train to trip before proceeding, thus ensuring that movements are conducted at safe speeds.
With a timed train stop, the trip arm 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 one that is set, the trip arm remains raised and trips the train to a stop. If the train approaches at a speed equal to or lower than the set speed, the trip arm 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 the trip arm is lowered on a yellow signal.
For sections of track with lower speed limits (15–20 km/h) a simpler construction is also used. The trip arm rotates freely on a horizontal axis with a counterbalance attached to its lower end. If a train's speed is low, the arm will be rotated by the trip cock with a force insufficient to initiate braking. But if its speed is too high, force will be large due to the counterbalance inertia, causing the brakes to be applied.
With fixed train stops, the trip arm cannot be lowered. Fixed stops are positioned close to the end of a dead-end track, to stop a train before it runs out of track. They may also be used at the end of track sections beyond which certain trains should not pass, such as the end of electrified territory (e.g. Hamilton, NSW), or to test the automatic brake and tripgear of trains departing certain locations, e.g. storage sidings.
A fixed train stop that is the last one on a running line in the reverse direction may, despite its name, be suppressed, as is the case with the associated "Fixed Signals" on the Sydney network. Suppression is needed because in Sydney, the rear trip cock on a train is always lowered, while in Melbourne, by contrast, suppression is not required because the trip cock at the rear of a train is always raised clear of any wayside trip arm.
In 1901, Union Switch and Signal Company developed the first automatic train stop system for the Boston Elevated Railway. This system was soon adopted by the New York City Subway and other transit systems in the United States. Similar systems were installed around this time on the London Underground system.
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 Toronto Transit Commission Subway is another system where train stops are used.
Train stops on London Underground lines are gradually being phased out in favour of ATP and distance-to-go signalling. Train stops remain standard equipment on all RailCorp metropolitan passenger lines in New South Wales, and on the electrified suburban railway system in Melbourne, Australia.
When trains operate in the reverse direction, they may "back trip" on train stops applying to the normal direction, which is a nuisance. This may be avoided in one of three ways:
The mechanical trip are is proved depressed in the green light of its associated signal.
The trip are is proved at normal (red) in the track circuit following that signal.
The proving switch detects the actual arm of the train stop, and breakage of that arm centres the switch neither depressed nor normal.
The well known car chase scene in the motion picture The French Connection shows a train stop being struck by a New York City elevated subway train as it approaches the train ahead of it on the same track. Although the moving train hits the train stop and begins to slow down, for the purposes of the film sequence, it was supposedly traveling too fast to reach a full stop before colliding with the train ahead.