OK I’ve been a lifelong railfan but have never understood what may indeed be a very simple concept about train brakes. Without much thought, my assumption has always been that an absence of air pressure in the brakeline puts a car’s brakes into “locked on” mode. This could not be correct, though, could it? I imagine that if a train loses it’s air while in motion, the result of every wheel locking up would be disastrous. So is air pressure required both to release and apply the brakes?
Many thanks for answers in advance