"Unlike cars, trucks and trains use power to deactivate their brakes"
A coach or large truck has dual chamber brakes , known as "spring brakes".
With about 60# air pressure the spring is held off and any application of the in cab brake tredle will supply graduated air to slow the vehicle.
It takes this air pressure to control the vehicle.
Many OTR trucks have a dash gauge for break line pressure
Should ALL air pressure be lost the spring will actuate and slowly stop the truck/bus.
The spring brakes are only on the rear axle (s) so the stop is not violent.
To tow the truck , either a special tool to cage the springs , or an air line from the tow truck is required to release the springs..