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The problem of determination of the accurate distribution of the return current in AT (autotransformer) electric traction systems (supplied at 2×25 kV) for High Speed Railways is considered. The path of the traction return current flowing from the rolling stock axles back to the supply (i.e., substation) is composed of the traction rails and additional earth potential conductors. The overhead supply conductors in contact with the train pantograph are connected to a symmetrical circuit (the feeder) with the purpose of current balancing. This arrangement and its influence on current to earth are considered: the return current divides among rails (as signalling disturbing current) and earth, depending on the value of the electric parameters of the system and the earth and on the circuit arrangement and on the relative position of system devices. The amplitude (as a percentage of the total return current) of the disturbing current may be high enough to cause interference to signalling. This work investigates the behavior of the return current in AT electric railway systems, on the basis of a reference system for the variation of the most important electrical parameters.