Train stops, also known as trips, tripcocks and tripstops, are devices fitted to most signals on the Sydney Trains metropolitan and intercity network. They help to reduce the consequences of SPADs (signal passed at danger) by stopping the train at a red signal.
Trains stops are small lineside devices with a movable arm (powered by electricity or air). They are always mounted on the left side in the direction of travel.
Train stops are connected to a signal. When that signal shows a red signal, the trip arm will come up. If a train tries to go past the signal, the arm will strike the trip cock on the train, which then will apply the emergency brakes on the train. Because the emergency brake has been applied, the train stops and can't move for a short time, because the brake pipe has to refill.
In some cases, when the signal is displaying a shunt ahead aspect, the train stop will also not drop. This ensures that the train moves past the signal at a slow enough speed, ready to stop short of an obstruction. This is because the shunt ahead indication does not necessarily indicatate that the line ahead is clear.
One disadvantage of train stops is that they will only apply the brakes after a train has passed a red signal. This means the train will still pass the red signal and can still potentially collide with another train, particularly if the train was travelling at high speed. Because of this, other safety devices are also used to mitigate the consequences of SPADs, in particular, catch points (which will derail a train away from danger) and ATP (automatic train protection, which will stop the train before it passes the red signal).
Additionally, like most safety devices, they are not completely failsafe. In the case of a signal failure, drivers will be instructed to trip past failed signals (strike the trip arm at low speed and continue once the brake pipe is full). This can have disasterous consequences if drivers are not careful, such as in the Glenbrook Train Crash, when a V set struck the Indian Pacific after tripping past a failed signal.
Special Train Stops Edit
Low speed train stop Edit
This is a type of train stop attached to a signal that can display a low speed indication. The trip arm still works normally while the signal is at stop. However, when the signal is changed to low speed, the arm will stay up. As the train approaches the signal, sensors will measure the speed of the train, by counting how long it takes for the train to pass a certain section. If the speed is under 25km/h, the trip arm will drop and the train will be able to pass. However, if the train is over 25km/h, the train will strike the trip and be brought to a halt.
Intermediate trips Edit
These train stops are found along the length of a platform, seemingly not connected to any signal. However, they are connected to the signal at the end of the platform. When the signal at the end of the platform is at stop, all the trips will come up and act like a low speed trip. They will force the train to slow down for the red signal. The last trip, closest to the signal, requires a speed of about 4km/h.
Both low speed and intermediate trips allow for trains to run closer together, hence they are only found on core sections of the network such as the City Circle. They allow for increased safety as they will stop a fast moving train much earlier than if there was only one train stop.
Fixed train stops Edit
These are train stops that are stuck in the up position, to protect dead ends. They normally have the shape of a triangular prism.
Reverse Direction Edit
When the train is going backwards through a train stop for any reason (e.g. bidirectional signalling), the train stop will come down even if it is at stop. This is because there may be other driving cars on the train facing the right direction to be struck.