Abstract: This invention applies to an automatic train control system. A low power radar mounted beneath a train interrogates simple passive transponders mounted between the rails. Each transponder includes a reflective delay line. The radar has a number of range gates. The transponder delays are arranged in a simple code which provides information as to the location of the train along the track. The outputs of the range gates can be fed to a simple computer which can also receive wheel revolution pulses from an axle mounted transducer. The computer can then calculate the distance travelled at any given moment to within one meter from the last transponder over which the train has passed.