Train air brakes do fail safe, the brake is applied by reducing air pressure in the brake pipe. The greater the reduction in the brake pipe the greater the pressure sent to the brake cylinders from the auxiliary reservoir on each vehicle. The device that does this in Europe is a distributor, in North America an updated version of the triple valve. In a single piped train the auxiliary reservoir is charged by the brake pipe when at it's full pressure (72.5psi in the UK, 90 odd psi in NA). When the distributor/triple valve "sees" a reduction in brake pipe pressure air from the auxiliary reservoir is allowed to enter the brake cylinder proportionally. When the brake pipe is reduced to zero the full pressure is applied to the brake cylinder. The problem then is that it will leak off over time. In the UK it is stipulated that enough handbrakes are applied to hold the train when the air has leaked off. What puzzles me is unless the train was dual piped (with a main reservoir pipe feeding the auxiliary reservoirs) and I have never seen a North American freight train that is, just leaving the loco running would only hold the brakes on the locos, not the train, unless the brake pipe was periodically recharged and destroyed, to make sure that all the brake cylinders still had air in them. Sufficient handbrakes should have been applied to hold the train, before it was left unattended, even if that was nearly all of them.
Cheers,
Peter.