Absolute Block being something entirely different, where there are no track circuits or axle counters. Only one train at a time is allowed between any two signal boxes with each train checked as it leaves a section to ensure it is complete and none of it remains in the section, the tail light being the indication that a train is complete. If an accident were to occur in an absolute block area which blocks the opposite running line then placing detonators may be the only method of protection, since a train on the opposite line may already have entered a long block.
Absolute Block sections do not normally have track circuits in the section.
Track circuits
may sometimes start at the distant signal, but in most situations are confined to the area from the Home signal berth to the Section signal.
Where there is an Intermediate Block section, then the track circuits will extend from the berth track circuit at the Home signal to the IB Section signal.
In the event of an incident affecting an adjoining running line, protection is by means of NRN, and then detonators.