A traction system for vehicles comprises a first power source of the non-reversible type, a second power source of the reversible type and a transmission which is connected to the power sources and which includes a first differential device. The transmission further comprises a speed variator which is interposed between the first source and the first differential device and which comprises a continuous speed variation device, and a second differential device. The continuous speed variation device is of the toroidal friction wheel type and comprises a lateral motion input disc, a lateral motion output disc and at least two idle oscillating friction roller members. The lateral motion input and output discs have a friction surface of toroidal shape and the idle oscillating roller members have a friction surface shaped in the form of a spherical dome.