It is possible under the right circumstances, for an internal leak within the car's brake system valves to allow air to internally leak back into the train line. If the train line (brake pipe) pressure increases by only a couple of pounds, the brakes will release. It has happened many times.
I personally watched it happen to us once on the Ohio Central when we bottled the air on a cut of cars to "save time." In less that two minutes the brakes released. We were on a slight grade and the cars started slowly rolling away. Fortunately we were in position with the engine to catch them quickly before any damage was done. But there was no time saved, that's for sure!
If the Strasburg does this on a regular basis and has done it for years without incident, that's fine. The siding at Leaman Place may be level, so even if the brakes did release, the cars won't go anywhere. As I said in my original post about this, I don't know if this is a Federal rule or a local rule. If it's not prohibited on the Federal level, then they aren't breaking any rules. However, I'm with Big Jim when he stated above, "That does not make it a safe practice!"
