A new architecture for an ATC (Automatic Train Control) system using digital track circuit signals is presented. Its key ideas are the following. (1) Digital track signals expand flexibility of the ATC system design without compromising safety. In comparison with the present analog type ATC typically in use, the probability of the new system being down is expected to drop below one third, and the space occupied by equipment for the new system is halved. (2) Jointless track circuits realize an overlay track circuit compatible with current track circuits. (3) The unity control zone, consisting of a few stations, can function whether a neighboring zone exists or not. (4) The multi function ATC device on board can process present and new ATC signals simultaneously. (5) Items (2), (3) and (4) allow step-by-step system construction.