to proceed it pivots down. A trip cock is mounted on the corner of the truck (bogie) of each car. The placement of the trip cock depends on the division the train runs on; some rolling stock is equipped with a trip cock on both sides. The train stop can be identified on the tracks because they are painted yellow for evidence if it has ever tripped a train. Sydney now uses the London Underground type J trip arm called the JA, except that it is mirror-image mounted on the left hand side of the track instead of the right hand side. A nearly identical system is used in Wellington. Toronto Subway also uses a T-shaped metal bar that rises from the floor except it is always on the right side of the track for all rolling stock because the trip cock is placed on the right side for all rolling stock. Reverse direction When trains operate in the reverse direction, they may "back trip" on train stops applying to the normal direction, which is a nuisance. This may be avoided in one of three ways: In Melbourne the rear trip cock is raised, although this runs the risk of runaways from terminal stations at high elevations. In Sydney, the rear trip is always lowered, and trip arms in the opposite direction are "suppressed" so that they do not engage the rear trip cock. Trip cocks on intermediate cars, if any, are always raised. In New York, when the signalling system clears motion in one direction (on bi-directional track or through an interlocking), it will automatically drive the trip arms which apply to the other direction. This is necessary because trip cocks are located along the entire length of trains in New York, not just the lead car. On the London Underground there are many train stops on the Piccadilly line facing the opposite direction of travel. This is because they protect engineer's trains that often travel in opposite direction during engineering works. Proving The mechanical trip arm is proved in the lowered position before the red light of its associated signal is extinguished. The trip arm is proved in the raised position before the track circuit following that signal is indicated as unoccupied. The proving switch detects the actual arm of the train stop, and breakage of that arm centres the switch neither depressed nor normal thus alerting operators