A colloquial term applied by crews and railfans to equipment featuring a dark, severely angled or hooded front end resembling the famous movie villain's helmet. In North America, it often refers to certain wide-cab diesels or specific commuter rail cab cars with sloped fiberglass noses. British railwaymen use the same nickname for the Class 460 electric multiple units due to their heavily raked front profiles.
Heard on the job
They assigned us that Darth Vader cab car for the evening commuter run.