[Pontiac]

https://www.popularmechanics.com/technology/a9176/understanding-a-runaway-train-how-do-air-brakes-work-15678938/

I read this article and was surprised.

I long thought that Train brakes work like a Semi-truck’s brakes which requires air pressure to release the brakes.

Without air pressure the brakes fail to the BRAKING position.

[old curmudgeon]

If train brakes worked that way, they could no longer hump cars when making up trains.

I suppose they still hump cars.