Air pressure is required for the brakes to be applied, insofar as if you bleed all the air off the cars their brakes will release. However, air is stored in re

Air pressure is required for the brakes to be applied, insofar as if you bleed all the air off the cars their brakes will release. However, air is stored in reservoirs on the cars, and once charged when the air pressure in the brake pipe that goes down the train is reduced below the reservoir pressure the brakes apply (by using air stored in the local reservoirs). As such, the brake pipe is simultaneously an air charging system and a signaling system. It's also fail-safe for normal operation since a train that breaks in two will come to a stop. Passively-applied brakes would break a ton of standard railroad operations which depend on cars rolling free, such as in "hump" classification yards which use in-track retarders to control speed, and in other situations such as "kicking" cars into tied-down cuts.