A power door operator for transit vehicles utilizing a rodless pneumatic cylinder as a prime mover in opening and closing doors in the sidewall of a transit vehicles. Door movement having a controlled, predetermined door edge force is achieved through mechanical coupling an external piston of the rodless cylinder and vehicle doors through and endless toothed belt. Magnetic coupling between internal and external pistons of the cylinder provide controlled force having a maximum or breakaway value applied to the operated door. A novel reaction lock operated by admission of pressurized air to the cylinder latches or unlatches the operated door in its closed position subsequent or prior to door closing or opening. Control of door motion at the ends of its travel is achieved through the use of a novel differential area internal piston in the pneumatic cylinder. Available internal piston force is modified for predetermined piston positions on approaching either end of the cylinder through the use of a sliding rod contained by the pressure sensitive internal position. Movement of the sliding rod adjust the effective piston area so as to modify available force to controlling door motion. An additional embodiment of the invention disclosed utilizes direct panel locking means independent of the operator drive belt.