This happens silently and goes unnoticed until some NFS client attempts to lock a file on that server. If it is not, it is probably necessary to restart the service manually. The area had a large and complex track layout controlling several main and secondary lines and freight routes to the local dockyards. On that day two trains met head on, one having been incorrectly routed onto the wrong track. To understand the accident it is necessary to realise that signalling safety depends on more than just the interlocking frame.
Additional equipment detects the presence of a train to prevent a switch being moved under it.
These days this is done using electrical track circuits, in which the presence of a train is detected by the short-circuit through the axles from one rail to the other, but in mechanical systems this was not always present and various mechanical detectors were used. In the Hull accident, the signalmen were concerned not to delay a late running train and one of them reset the signal cleared for train A after the locomotive and three coaches had passed it. At this point the locomotive was still thirty feet from the trackside treadle which would lock the next switch during passage. During the time the train took to reach the treadle, less than a second, it had effectively disappeared from the logic of the interlocking frame. The interlocking that would have prevented this had been disabled because the first signalman had been too quick in cancelling the cleared signal, releasing the interlock on the switch, and the trackside detection of the train had not yet been triggered.