Re: NNG - SLRG's busy and "interesting" weekend.....

Dw...IIRC, truck brakes are sprung to grab when air is absent, as a safety measure.

Railway equipment has no such setup.
Air needs to be pumped into each car's reservoir, ready to apply brakes via each car's sensing "triple valve" when trainline air pressure is reduced or disappears.
When reservoir air is gone, the only backup is the manual handbrake and/or a big wood chunk...If you're standing still.
To release air-applied brakes, air is pumped through the car's trainline, the triple valve then re-fills the reservoir and exhausts the air in the car brake cylinder.
Its piston will then retract via an internal spring, attached linkage will pull brake shoes/ calipers away from wheel treads/discs, ready for the next application.