Editing Train stop (section)

== Basic operation ==
[[File:Train-trip-lever.jpg|thumb|Train-mounted trip cock, located on the leading bogie of a [[Siemens Nexas|Siemens EMU]]]]

The train stop system comprises two basic components. One is the trip arm mechanism, mounted on the ground adjacent to the [[Rail tracks|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 [[Valve|cock]], which is connected either directly or electrically to the train's [[Brake (railway)|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 (device)|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 {{cvt|2+1/2|in}} above the top of the running rail when in the stop position, and lower to approximately {{cvt|1|in}} 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 {{Convert|100|km/h|4=-1|abbr=on}}.<!-- at the Berlin Ringbahn -->  Whereas later contactless train stops require a working receiver on the train to initiate the emergency stop (as for [[Pulse code cab signaling|PCCS]] or [[Indusi]]) the mechanical stops have initially been kicking the brakes directly.

Because the trip arm may hit the brake lever 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 (railway signalling)|overlap]] due to excessive speed. In order to externally control the speed of a train the operation of [[#timed train stops|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.