Abstract: A process for localizing a mobile unit on a railway network, with a system that includes: a sensor (R) in the form of radar having an antenna (A) which is integral to the mobile unit and that cooperates with fixed markers (RP) placed alongside the railway track; a shaping circuit for a signal issued by the sensor; the radar being operative for measuring the distance traveled or the travel time; a database (BD) which contains the description of a graph of which knots or nodes represent the fixed markers and branches represent the railway tracks that connect them; and a circuit (OT) controlled by the shaped signal, connected to the radar and to the database, for determining the position of the mobile unit.