Pneumatic door locking system
The pneumatic door locking system is used on rail vehicles to close the external doors. It works with compressed air and is operated either by a train attendant at one of the entry doors (using a square key switch) or by the driver from the driver's cab of the locomotive or control car . In modern vehicles , the closing of the outer doors is announced by a "warning sounder"; it is intended to help prevent people or objects from becoming trapped.
The types of pneumatic door locking systems are listed under door control .