Track-circuit signaling on electrified roads

Railroads signals may be divided into three general classes according to their functions: (a) Those which confer rights on trains, or restrict their rights, known as train-order signals; (b) Those which designate the route a train is to take, and insure the safe condition of all switches and opposing signals on such route known as interlocking signals; (c) Those which are used primarily for properly spacing trains, known as block signals.