Abstract: The system maintains the door of a carrier of goods, such as a truck or railway car, locked when the carrier is parked and inoperative. The system includes a latch mechanism including a bolt spring biased to a latched position and a piston and cylinder mechanism coupled to the latch mechanism in such a way that air pressure applied to the cylinder moves the bolt to an unlatched position. The cylinder is connected by a conduit to a source of air pressure operated by the prime mover of the carrier and a valve mechanism is coupled to the conduit and is manually operable to vent, or supply air pressure to, the cylinder to operate the latch mechanism. When the prime mover is inoperative or disconnected and the carrier is at rest, air pressure is vented from the cylinder to latch the door to prevent theft of goods.