NOVELTY - The on-board power unit (5) comprises an alternator (4) coupled (3) to a petrol or diesel engine (1). Its rectified output (42) is connected through a change-over contactor (22) to the traction motor (6), which drives road wheels (7) via a gearbox (8).    USE - Hybrid propulsion and control system for public service vehicles.    ADVANTAGE - The on-board prime mover generally operates at the most advantageous torque and speed for a given tractive power demand, maintaining the latter at a consistent level without fluctuations due to arbitrarily imposed ancillary loads. Trolley operating mode allows battery weight to be reduced or eliminated.    DETAILED DESCRIPTION - The vehicle also carries trolley poles (11) with collectors running on an overhead DC catenary distribution system (10), from which, when a change-over switch (21) is operated, actuating the contactor (22) via a supervisory controller (9), power can be drawn by the traction motor, leaving the on-board unit isolated. Thus configured, the vehicle may or may not carry an auxiliary battery (12), rechargeable from the external system under the supervision of the main controller (9). Engine and alternator control units (2,41) are associated (50) with the electronic supervisory controller (9). This receives input from an accelerator pedal position sensor (20), which conveys the driver's moment-to-moment power requirement. The controller imposes this demand, with an addition covering all applicable ancillary loads, and taking due account of motor speed, on the power unit, calculating the optimum operating torque and speed from the known characteristics.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram showing the interrelation of power and control components.    IC engine (1)    Drive-shaft (3)    Alternator (4)    Traction motor (6)    Driven wheels (7)    Gearbox (8)    Supervisory control unit (9)    Overhead contact wires (10)    Trolley poles (11)    Auxiliary battery (12)    Accelerator pedal position sensor (20)    Contactor (21)    Selector switch on-board/external (22)