From time to time, failures will occur in signals, which can be of two types:
When a failure is detected, the failed signal will show as unlit and the interlocking will set the preceding signal to Red. The failed signal will remain unlit regardless of the state of the signals ahead of it.
If and when a train approaches this (first) red signal, the driver will call in after a couple of minutes to report that (s)he is waiting at the signal.
The signaller will normally then authorise the driver "to pass the signal at red", having set the required route. This is to ensure that the points are correctly called and locked in the correct position and that the route is clear of conflicting movements, even though the signal won't clear.
The train will then proceed to the failed signal, whereupon the procedure will need to be repeated.
Sometimes, only individual aspects will fail. In this case the effects of the failure depend on which aspect has failed.
For example, if the green aspect is out, the signal will correctly display R, Y, YY (if 4 aspect) only. If the next signal (in advance) is showing YY, the failed signal will be unable to show the G aspect and will be considered failed, causing the previous signal (in the rear) to immediately revert to red; this could cause a train to receive an Adverse Change of Aspect. The signaller will therefore have to be careful in manipulating the various signals to try to prevent this happening.
For example, suppose that the signals in order are 1, 3, 5, and 7, and the green aspect at 3 has failed. The signaller could therefore hold either signal 5 or signal 7 at red until each approaching train has passed signal 3. The choice of 5 or 7 would depend on which ones are controlled - 7 is better because it allows 3 to show YY rather than Y. If, on the other hand, it is the yellow lamp that has failed at 3, the signaller would hold 3 at red (or 1 if 3 is an automatic with no A button) until both 5 and 7 can be cleared allowing 3 to show green.
However, if it isn't possible to prevent the train approaching a failed signal, you will have to instruct the train to Pass at Danger as above.
Shunt signal aspects are not necessarily 'proved' lit, and so a failure will not necessarily be shown on the panel. Therefore your first indication of this failure may be when the driver calls in waiting at red, even though the signal shows white/clear on the panel.
Again, at this point the driver is authorised to "pass the signal at danger"
Last edited by GeoffM on 15/09/2016 at 03:01