ac induction type traction motors used for propelling a transit vehicle not shown and the dc source 12 comprises a wayside power distribution system including either a third rail or an overhead catenary with which a current collector on the vehicle makes sliding or rolling contact in fig1 the relatively positive line 17 represents such a current collector and the negative line 19 represents a conductor in contact with a grounded rail serving as the other terminal of the dc source alternatively the power conversion system 10 on board the vehicle may be connected to the source via a two wire trolley in contact with two parallel conductors of an overhead catenary the magnitude of the unipolarity voltage between the lines 17 and 19 is typically in a range from 600 volts normal to 800 volts maximum and each of the motors 16 and 18 typically has a full load rating on the order of 300 horsepower more or less the power conversion system 10 includes a controllable converter 20 which in the illustrated embodiment of the invention is a voltage source inverter having a pair of dc terminals 22 and 24 on its source side and a set of three ac terminals 26 28 and 30 on its motor side the dc terminal 22 is connected via a conductor 40 to the lines 17 of the positive potential and the terminal 24 is connected via relatively negative conductors 41 and 42 to the other lines 19 of the dc power source 12 the conductors 40 42 thus serve as a dc link between the source 12 and the inverter 20 the ac terminals 26 28 and 30 are respectively connected to.