No, you've got it right. Confused? :unsure:Perhaps I've got it completely backwards, but don't the brakes apply as the air pressure decreases?While it's true that the air brakes were applied from the moment that they separated the train, the conductor set the hand brake because over time the remaining air will slowly bleed off and eventually the brakes would release. Depending on leaks and other factors, it could take many hours or just a few. So by setting the hand brakes on one or two cars, the conductors ensure the utmost safety in case the switching ops take too long.
As noted, a runaway train would not be a good thing.
However, the way things are designed, once the air totally drains from the resevors of each car, the brakes will no longer work. It's a rather complicated system, that requires that air pressure be maintained at all times to make things work. See this article for a run down on why initially no air coming through the main hose applies the brakes, but after several hours of no return of air pressure in the hose the air holding the brakes will bleed off and the brakes will release.