Green’s Dictionary of Slang

(US) a motorcar.

1909		Boston Herald 29 July 3/1: A big chug wagon jumped away from the Senate office building late this afternoon and snorted off toward Fort Myer [DA].
1910		E.S.Field Sapphire Bracelet 35: Besides, your chug-wagon wears a New York number [DA].
1929		W.R. Burnett Little Caesar (1932) 253: Chugwagon, motor-cars.
1961		C. Barks [comic bk title] Chugwagon Derby.
1992		(con. 1900s) J. Rodda [bk title] From chugwagon to parkway: The development of automotive transportation in Arizona, 1900–1940.