A locking mechanism (100) for use with a pneumatic cylinder/differential engine for a power-operated door including a locking rod (20), a plunger (24) associated with the locking rod (20) to cause extension and retraction of the locking rod (20) with respect to a door opening/closing gear (46), and a spring member (34) associated with the plunger (24) for maintaining the plunger (24) and the locking rod (20) in an extended position during a door closed position. An aperture (46a) is located through a sidewall portion of the gear hub (46b) which is capable of receiving an end (20a) of the locking rod (20) when the locking rod is in an extended position to lock the door in a door closed position. The invention also includes an emergency door opening mechanism enabling manual opening of the doors in case of an emergency.