Train stop

When the signalling system determines it is safe for the train to proceed, the motor drives the trip arm down to the lowered position.

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).

There are three types of train stops: The trip arm is raised whenever the signal is not displaying a proceed aspect.

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

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.

In 1901, Union Switch and Signal Company developed the first automatic train stop system for the Boston Elevated Railway.

Its widest application, therefore, is on underground rapid transit lines, where conditions that might interfere with proper operation are readily controlled.

It is also not suitable for high-speed operation, as the maximum speeds permitted often do not exceed 115 km/h (61 mph), citing risk so high impact forces breaking the arms.

Train stops on London Underground lines are gradually being phased out in favour of ATP and distance-to-go signalling.

This may be avoided in one of three ways: On the London Underground there are many train stops on the Piccadilly line facing the opposite direction of travel.

When the signalling fails, etc., it may be necessary to "trip past" a train stop in the danger position, in accordance with the rules.