Loading ...…is a royal pain for the crew. Every car must be hand cranked to manually tighten the brakes. And later on, every car must be manually loosened also. Imagine walking a 100-car train, cranking on each brake wheel. It can take hours. Air brakes on a train depend upon a pressure tank of stored air on each car to ‘set’ the brakes for that car when the main braking air line is disrupted like a disconnect or derailment. Setting the emergency brakes involves just discharging out the air line from the locomotive end, and all the cars set their brakes from stored air tanks when that happens.
If the train parked at Field started to roll on it’s own with the crew onboard, that tells me the air brakes failed. In extremely cold weather I can see that happening as air seals leak, tanks lose pressure, and the brakes release. It takes a long time to re-pressurize the brake lines and all the car tanks on a long train. Maybe the crew thought they could repressurize the brakes before exceeding the speed limit, but by then the brakes would not be effective at excess speed.
Hand brakes are not normally set during a crew change as the train is considered ‘attended’ as one crew leaves the cab and another climbs aboard immediately. It can be as short as a 5-minute stop for a real hotshot train.