Nowadays, in several European Railway networks, the old DC supply systems are severely limited by line-voltage drops. In some sectors, it is not possible to increase traffic or to operate locomotives at their nominal ratings. To increase the capacity of a DC supply system, without additional substations, the authors propose a new three-wire supply system which restores the overhead line voltage with multilevel DC choppers operating as voltage boosters. Circuits using a third wire with a negative or a positive polarity are considered and compared. Case studies are presented for the French and Italian rail networks. Finally, experimental results performed on a test platform of the French railways, are presented in the case of a double 1.5 kV supply system.