directly, there have been installations where every wagon of a train has a separate trip cock on each bogie that has brakes. Hence in some installations the trip arm will be folded back to the safe position only a few seconds after a signal has turned red in order to reduce wear (which is an actual security risk as shown in the 1995 Russell Hill subway accident). Because of wear the trip arms for the opposite direction on the same rail will be actuated along with the trip arm for the nominal direction. Like many early systems a mechanical train stop does not inherently control speed. There were accidents where trains had overrun the overlap due to excessive speed. In order to externally control the speed of a train the operation of timed train stops has been deployed as it is widely used on the New York City Subway, where a second trip arm at a distance is folded down only a few seconds after a train has passed a check point. These expensive speed control systems are generally phased out in favour of a cab control computer that can continuously check a braking curve while approaching a signal. 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 from passing a point Trip 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. Timed train stops 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