The dead man's switch only works while the engine is running. The only way to secure the train is to set the manual brakes and chock the wheels. That way if the air bleeds down on the air brakes you're not depending on them alone to immobilize the train.
I had been under the impression that such brakes were “Normally Closed” systems, where lack of air pressure resulted in the railroad car applying it’s brakes automatically. That way, if a RR car is decoupled and loose, it naturally brakes. Am I mistaken.
(Never worked as a Friction Engineer.)