Train exiting the Tehachapi Loop - GIF

[Crusty]

The air brakes on trains (and trucks) are operated by pressure pots which are spring loaded with very strong springs to always apply the brakes. In the pots are two diaphragms that communicate with the air systems - one system in which pressure is applied to compress the spring for normal operation and variably bled off to apply variable braking, and the other functions as part of the emergency system so that if primary air pressure is lost the spring applies full braking. Those springs are bad enough that it's a somewhat tense operation in a shop to disassemble one to change the diaphragms.