Green’s Dictionary of Slang

(US tramp) a freight wagon.

1930

G. Milburn ‘The Swede from North Dakota’ in Hobo’s Hornbook 139: A ‘Jim Hill wagon,’ of course, is a freight [...] I buy me a suit, I buy me a bottle, / Dress me up way out of sight; / Yump on the tail of a Yim Hill wagon, / Yesus Chreest, I feel for fight.