MR stands for main reservoir, the primary high-pressure air storage tanks on a locomotive. Supplied directly by the engine's air compressor, these tanks typically hold air between 130 and 140 psi. This stored volume feeds the brake pipe to control the train, operates the locomotive's independent brakes, and powers auxiliary pneumatic systems like the horn, bell, and sanders.
Heard on the job
We've a bad leak on the second unit because the MR pressure won't pump up past ninety pounds.
Insider tip
Engineers keep a close eye on the MR gauge; if it drops below the regulating valve setting, the brake pipe cannot recharge after a brake application.
Also heard as
- Main reservoir