locomotive engineer; engine driver
Refers specifically to the person who operates a steam locomotive or train engine. A somewhat dated term; modern usage often prefers 機関士 or 運転士.
His grandfather was a locomotive engineer on a steam train.
運転士 is the general term for a train driver or operator today, whereas 機関手 is an older, more specialized term.
Compound of 機関 (engine, locomotive) and 手 (hand, person in a role). The term reflects the historical role of manually operating a steam locomotive.