Abstract: The invention is a conveyor control system having self-propelled vehicles having wheels, each driven by a motor (14) to move along rails. The system includes power rails (18) for supplying power to the vehicle (12) and at least one command rail (20, 21) which transmits a command signal indicative of requested speed. The command rail (20) includes of isolated sections of rail for carrying different command signals along each isolated section to enable the vehicle (12) to travel at different speeds dependent on which section of rail the vehicle is within. The command signal is in the form of a digital half-wave signal on two rails (20, 21). The digital signal is in the form of half-waves each representing a bit of binary coding. The digital signal may include three-phase half-wave coding in order to obtain four bits of coding from a single rail. Each vehicle (12) includes a control circuit (22) which interprets the command signal to drive the vehicle (12) at the requested speed.