Green’s Dictionary of Slang

(orig. US black) a car.

1965		S. Yurick Warriors (1966) 25: They hooted and laughed at the smooth black mass of gleamy Detroit Iron.
1972		(con. 1916) G. Swarthout Tin Lizzie Troop (1978) 28: Having observed them operating Detroit junk, he wanted to see how they handled horseflesh.
2000		(con. 1940s–60s) Décharné Straight from the Fridge Dad 44: Detroit disaster Automobile.