Basic Operation
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 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.
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