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. 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 failure of the electric or pneumatic power supply or of the motor driving the trip arm. 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 above the top of the running rail when in the stop position, and lower to approximately below the top of the running rail when clear. The time taken for the arm to rise or be lowered is approximately two seconds. Restrictions The mechanical train stops have been found to be relatively safe with their historic models from the 19th century to be still in use in the 21st century. Because of the quadratic force increments in relation to speed these mechanical systems are restricted to low speed applications. Their continued usage can be found in urban rapid transit systems with trains making no more than 100 km/h. Whereas later contactless train stops require a working receiver on the train to initiate the emergency stop (as for PCCS or Indusi) the mechanical stops have initially been kicking the brakes directly. Because the trip arm may hit the brake lever