Revolutions per minute, the metric used to measure the rotational speed of a locomotive's diesel engine crankshaft. In North American diesel-electric locomotives, the engineer controls engine RPM using the throttle, which is divided into eight notches. Higher RPM spins the main alternator faster, generating more electrical power for the traction motors.
Heard on the job
When you pull the throttle out to notch eight, the prime mover winds up to its maximum RPM and the whole cab shakes.