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The main purpose of any railway signalling system is to allow safe operation of the railway. This page explains how signals are `interlocked' with each other to prevent conflicting moves being signalled. On a model railway the traction current to the track can also be controlled by the signalling system, by isolating a short length of track immediately before a Stop signal when the signal is showing the Danger aspect ensuring trains cannot run past the signal at danger. (SPAD) If the train is driven towards the signal correctly is should stop before the short dead section. If the train is driven towards the signal at speed the train will stop very rapidly just before the stop signal. Some locomotives fitted with flywheels are able to coast some distance so the controlled section of track should extend far enough past the stop signal that flywheel drive locos may be able to overrun the signal (technically still a SPAD) but will stop in the dead section before reaching the normally powered track. The dead section is reconnected to the section After the stop signal when the signal is cleared, allowing the train to continue past the signal. Using signal controlled isolated stop sections may not be suitable if running multiple units with through connections/power collection on both ends/all wheels, due to the length of their pickup `footprint'.
All signals at Danger. If the section beyond the Advanced Starter is occupied by a train the Advanced Starter is held at Danger until the section becomes clear. The Main Starter and Loop Starter are also held at Danger until the Advanced Starter is changed to the Clear aspect.
The section beyond the Advanced Starter becomes Clear, the Advanced Starter is then changed to show the Clear aspect. The points are set for the Mainline, this unlocks the Main Starter signal which can then be changed to show the Clear aspect. A train on the mainline can then proceed safely. The Main Starter signal is interlocked with a detector on the points ensuring the Main Starter can only show Clear aspect when the points are set to the mainline. The Loop Starter is also interlocked with the point detector to ensure that whilst the points are set for the mainline the Loop Starter signal can only show the Danger aspect and cannot be changed.
The points are set for the Loop or converging line, The Main Starter is locked at Danger whilst the points are set for the Loop, the Advanced Starter is then changed to Clear aspect followed by the Loop Starter being changed to Clear allowing a train to proceed from the Loop line onto the mainline safely.
Signals without interlocking,
A disaster waiting to happen.
This is a potentially disasterous situation that could easily arise with individually controlled signals but would be prevented by interlocking the signals and points. The points are set for the Mainline, Both the Main and Loop Starter signals are showing the Clear aspect. A train passing the Loop Starter would be derailed on the points or could collide with a train stopped on the mainline at the Advanced Starter. When the Advanced Starter is showing Danger, Both the Main and Loop Starter signals should be locked at danger until the advanced starter is showing the Clear aspect. Only then should the Main Starter `OR' Loop Starter be cleared dependant on the running position of the points.
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