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[quote user="Jerry Pier"]The Brake Pipe on the Quebec train had 171 possible leakage points, ie; at each hose coupling. Leaks are worse at low temperatures but you can walk a standing train at any time and probably hear air hissing out at one or more gladhands. Pressure maintaining takes care of this as long as Brake Valve handle is in the right position and the compressor is providing air. When the locomotive is shut down, you are at the mercy of the main reservoir volume which is finite.[/quote] Jerry, Thanks for your explanation of the cylinder leak down.  As I understand it, you are ruling out that kind of leakage playing a role in the MM&A runaway because sufficient leak-down would have taken a lot longer than just the hour or so that it took for the train to start rolling. But I have some questions about the part that I have quoted from you above.  I understand the issue of trainline/brake pipe/gladhand leakage, but how exactly could that have played a role in the runaway? As I understand it, the trainline was at reduced pressure with brakes set.  If there was trainline leakage, and if the engine shut down, the trainline might have leaked down to zero.  But so what?  Would you not still have air pressurizing all the brake cylinders of the cars? One or more others have given the explanation that the only braking that would have been reduced or eliminated due to leakage if the compressor shut down was confined to the independent brakes.
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