On every railroad I have ever worked for, bottling the air was against the rules. It may be a violation of a federal rule, too, but I’m not sure of that and I’m not where I could look it up.
The danger in bottling the air is that even though the train brakes were applied before the engine cuts away from the train, it is possible for those brakes to release. If only one car has an internal leak in the service valve, that can allow air to leak back into the brake pipe. With modern air brakes, just a 1.5 to 2 pound increase in the brake pipe pressure will release the brakes!
On the short trains that Strasburg runs, bottling the air would not save any appreciable time on the run around move. Strasburg is a well run outfit, and I doubt that they are doing this, especially with people on board those standing cars.
