Rule 55 essentially required a driver or other member of the crew to contact the signalman (generally by walking to the signalbox) if his
train was delayed at a signal for a significant time (usually three minutes). The rule was there to ensure the signalman didn't forget the train was there and set up a conflicting move. Failure to observe the rule led to accidents such as Hawes Junction (1910) and Quintinshill (1915), in both of which an express train was signalled through a section despite a stationary train standing at one of the signals. At Hawes Junction the forgotten train (actually a pair of locomotives) moved off when the signal cleared, but were quickly caught up and rear-ended by the following express. At Quintinshill a local train had been shunted to the "wrong" road to allow a faster train to overtake, but the signalman (who had actually arrived on that very train to start his shift) then forgot about it and signalled a train through in the opposite direction. The carnage was added to when the train that was to have overtaken the local appeared on the scene moments later.
In these days of track circuits, and power signal boxes miles from the signals they control, the rule is largely obsolete.