Power supply system for an electric train, comprising • at least one traction module configured for being connected to a catenary, the power supply system thereby being configured for receiving power supply from the catenary when the traction module is connected to said catenary, said traction module comprising ∘ a DC bus connected to the catenary, and ∘ a converter configured for adapting the voltage of the DC bus with a first side connected to the DC bus and a second side configured for powering at least one traction motor for driving the train, • an auxiliary load module connected to the DC bus and configured for powering auxiliary loads (6, 7) of the train when the power supply system is powered from the catenary, and • a battery configured for powering pre-selected auxiliary loads and the traction motor in the absence of power supply from the catenary, the auxiliary load module comprising at least one battery charger, the battery charger being connected between the DC bus of the traction module and the battery, and configured for charging the battery from the DC bus when the power supply system is powered from the catenary, characterized in that the battery charger is unidirectional and is configured for only allowing the flow of current from its input to its output, the input being connected to the DC bus and the output being connected to the battery, the power supply system comprising a support module (4, 4') connected to the battery and to the traction module, and configured for allowing the flow of current from the battery to said traction module in the absence of power supply from the catenary, the support module (4, 4') comprising at least one DC/AC converter (40, 40'), the DC side of the DC/AC converter (40, 40') being connected to the battery and the AC side of the DC/AC converter (40, 40') being connected to the traction module by galvanic insulation means of the power supply system, said galvanic insulation means comprising at least one first winding (11, 14, 43') connected to the traction module and a second winding (12, 42') connected to the AC side of the DC/AC converter (40, 40'). • at least one traction module configured for being connected to a catenary, the power supply system thereby being configured for receiving power supply from the catenary when the traction module is connected to said catenary, said traction module comprising ∘ a DC bus connected to the catenary, and ∘ a converter configured for adapting the voltage of the DC bus with a first side connected to the DC bus and a second side configured for powering at least one traction motor for driving the train, ∘ a DC bus connected to the catenary, and ∘ a converter configured for adapting the voltage of the DC bus with a first side connected to the DC bus and a second side configured for powering at least one traction motor for driving the train, • an auxiliary load module connected to the DC bus and configured for powering auxiliary loads (6, 7) of the train when the power supply system is powered from the catenary, and • a battery configured for powering pre-selected auxiliary loads and the traction motor in the absence of power supply from the catenary, the auxiliary load module comprising at least one battery charger, the battery charger being connected between the DC bus of the traction module and the battery, and configured for charging the battery from the DC bus when the power supply system is powered from the catenary, characterized in that the battery charger is unidirectional and is configured for only allowing the flow of current from its input to its output, the input being connected to the DC bus and the output being connected to the battery, the power supply system comprising a support module (4, 4') connected to the battery and to the traction module, and configured for allowing the flow of current from the battery to said traction module in the absence of power supply from the catenary, the support module (4, 4') comprising at least one DC/AC converter (40, 40'), the DC side of the DC/AC converter (40, 40') being connected to the battery and the AC side of the DC/AC converter (40, 40') being connected to the traction module by galvanic insulation means of the power supply system, said galvanic insulation means comprising at least one first winding (11, 14, 43') connected to the traction module and a second winding (12, 42') connected to the AC side of the DC/AC converter (40, 40').