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[quote user="tree68"]That assumes there are no leaks. [/quote] I would not say that it assumes no leaks.  It assumes an insufficient amount of leakage to release enough brakes to permit the train to roll. There are two leakage scenarios: 1)      The leaking of air pressurizing the cylinders individually on a case-by-case basis. 2)      The leaking of air pressurizing the cylinders into a closed trainline, thereby raising its pressure and triggering a one-time release of all the cylinders. What Jerry is referring to is item #1.  It you set out 20 cars and dump the air, all cars go into emergency with full braking.  Some of those cars might leak off quickly and release their brakes.  Other cars in that cut may hold their air for weeks.  But the point is that one leaking does not release all of the other cars.  It only releases the car that is leaking. So with scenario #1, if it is one car standing, and it happens to be one that does not hold the cylinder air very long, it will run away quickly.  But if that one car is in a cut along with 100 other cars that do hold their air for a week, that cut will stay put for a week.   The loss of air in that one car will not release the brakes of the other cars.   But in scenario #2, one car leaking can release all the brakes even in less time than it would take for that one car to leak down and release if it were alone.
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