Abstract: A system and method for movement of self-propelled vehicles in-line along a rail under external control is intended to ensure safe spacing between vehicles to avoid collisions. Each vehicle is driven by its own variable speed motor. The speed of the vehicles is controlled by a stationary cam extending beside the rail. The edge of the cam rises and falls in a predetermined manner and is sensed by a cam follower on each vehicle. The cam follower is coupled to a transducer which produces a control signal for varying the vehicle speed in response to the rise or fall of the cam edge. Each vehicle pulls along a tail which extends behind the vehicle and is guided for motion alongside the edge of the cam.