Green’s Dictionary of Slang

(US black) the subway.

[1939		T.J. Farr ‘The Language of the Tennessee Mountain Regions’ in AS XIV:2 91: iron horse. Train].
1977		B. Davidson Collura (1978) 128: The little group nervously took the Lexington Avenue Subway – the fuckin’ iron horse – getting off at 14th Street.
1998		Big L ‘Ebonics’ 🎵 The iron horse is the train.
2002		‘Touré’ Portable Promised Land (ms.) 157: We Words (My Favorite Things) [...] White trash. Ice grill. Iron horse.