Abstract: Two separately-excited d-c electric motors are used to drive the wheels of a traction vehicle. The magnitude of motor field current varies as a function of the value of a variable control signal, which value is representative of any deviation of the field current magnitude from a given reference magnitude. When electric braking of the vehicle is desired, the reference magnitude is normally determined by the difference (if any) between the actual rotational speed of the slower motor and a predetermined desired speed, whereby motor field current is varied as necessary to regulate the retard speed of the vehicle. Means is provided for automatically initiating the retard speed regulation mode of operation as the vehicle accelerates and the actual rotational speed of the faster motor approaches the aforesaid desired speed.