The aggressive acoustic and visual display of a diesel locomotive operating at maximum RPM under a heavy load. Railfans and crews use the term to describe the visceral experience of ground-shaking engine roar, turbocharger whine, and thick exhaust smoke when a train is pulling hard up a steep grade.
Heard on the job
Listen to the thrash on that lead unit as they try to drag those coal hoppers up the ruling grade.
Insider tip
This is primarily British railfan slang that has bled into North American enthusiast circles; working railroaders in the US are more likely to say the engine is screaming.