From time to time, track sections (including track circuits) will fail. As signalling logic is designed to be fail-safe, a failure will look to the rest of the system as if the track circuit is occupied, even if no train is present. This will mean that the protecting signal will revert to Danger (Red), preventing the passage of trains.
In order to keep trains moving, there are provisions to enable them to proceed, but as the normal protective systems are in a degraded state great care has to be used to prevent accidents.
When the driver of the first train calls in from the signal, (s)he is instructed to proceed past the red signal at a reduced speed, examining the line for any obstructions or defects that may be the cause of the failure, and being ready to stop short if necessary. On reaching the next signal, or another location as instructed by the signaller, the driver will report back to the signaller, and is then able to continue as normal. Alternatively, (s)he may be asked to repeat the procedure through the next signal section.
When a track circuit fails to clear behind a train or otherwise shows occupied, in order to establish if the line is clear, the line must be examined. Within SimSig, the line is examined by trains (however in real life, it can be examined on foot as well). Before the signaller authorises a train to enter a section to examine the line, (s)he must check the following:
When the driver calls the signaller, the signaller will instruct the driver to pass the signal at danger, to proceed at extreme caution, being prepared to stop short of any obstruction and report in at a location specified by the signaller, which normally will be the next stop signal that the train will come to. A speed restriction of 10mph is imposed when passing through a tunnel.
The driver will then report the state of the line on arrival at the specified location and subsequent trains may be instructed to pass the protecting signal at danger to pass over the affected section of line. Before any train is instructed to pass a signal at danger, the signaller must run through the above check-list to ensure safety. There are rules that permit normal working if the track circuit stays clear after the examination, but SimSig doesn't simulate the conditions for these rules to be applied.
It is not good practice to set a route through a track circuit failure. The route locking will hold (show white) ahead of the track circuit failure and may remove flexibility ahead of the track circuit failure. It may also lock points out in the overlap of the next signal ahead preventing moves from being made on lines that shouldn't be affected by the fault. Sometimes a track circuit will fail while a route is set and this will sometimes cause a major loss of flexibility, but that is just down to bad luck then. In real life, it is also bad practice since track circuits can suddenly start to bob and this could cause the protecting signal to clear or cause irregular aspect sequences.
If the track configuration permits and it is possible to divert a train onto another line so that it can run under the full security of the signalling system, then the signaller should do so. If doing so, consideration may be given to stations that the train is booked to stop at before the train is diverted. It is not normal to Single Line Work around a track circuit failure unless it is due to the track circuit being caused by an obstruction or a broken rail (which is not simulated in SimSig).
After a track circuit fails to clear or otherwise shows occupied, if the first train to arrive on the scene is on an adjacent line to the track circuit failure, the driver of that train is instructed to proceed cautiously and check the state of the affected line and report back if anything is seen amiss. Subsequent trains may run normally on the adjacent lines after this until the first train arrives on the affected line.
During the examination of the line by the first train on the affected line, trains on the adjacent lines may be signalled normally but the drivers must be informed of the circumstances and instructed to proceed through the area at caution.
In SimSig, due to limitation in the software, this is perhaps best simulated by stopping the train on the adjacent line, waiting for the driver to phone in, then clear the signal.
When a track circuit fails within an overlap, the signal next in rear of the protecting signal will also be held at danger. In this case, the driver is to be instructed to pass the first signal at danger, after the signaller goes through the above check-list. If this is the first train, then examining of the line here is not necessary until the train actually passes over the track circuit itself. Be aware that setting a route here may lock out the overlap beyond the track circuit failure and in real life is best avoided since a “bobbing” track circuit failure could cause irregular signal aspect sequences.
If running a retro timetable, Retro-rules could be applied at the wish of the host. These include the adjacent lines being blocked completely while the affected line is being examined and the ban on using passenger trains to examine the line in a tunnel unless it is established that the tunnel is clear (if necessary by someone walking through the tunnel first).