To achieve that braking safety, the subway’s signal architecture works around the principle of ‘two-block control.’ Every track on the subway is cut up into blocks, electrically isolated sections of line that can detect occupancy by sending current through the axles of passing trains. In two-block control, the signal system keeps at least one unoccupied block – really, however many blocks you need to provide that braking distance with a margin for error – between one train and the next.
