Absolute Permissive Block

Absolute Permissive Block
(APB) A designated section of track or tracks within which the movement of trains will be governed by block signals, whose indications supersede the superiority of trains.

As mentioned above, some Absolute Permissive Block (APB) systems provide full safety for opposing as well as following trains.

Signals usually operate as APB (absolute permissive block), so yellow indications appear automatically and opposing moves see solid red. Trains operate following signal indications. Authority by timetable disappears along with most train orders.

See also: Tank, Track, Throttle, Passenger car, Freight car