Green’s Dictionary of Slang

1. (US tramp, also crumby) the caboose of a train, i.e. that coach used by railroad workmen or train guards.

1922		M.E. Smith Adventures of a Boomer Op. 19: Riding in a ‘Crummy’ that had about as much ventilation as the inside of a vacuum cleaner.
1934		AS IX:2 Apr. 74/1: There was a hundred and thirty rattlers and a crummy on that thing and you should have heard the old hog wheeze as we went down to the station.
1935		E. Anderson Hungry Men 186: You wanna catch her back toward the crumby, though, ’cause there’s always a bunch of empties up forward.
1945		F.H. Hubbard Railroad Avenue 2: The crummy silently does stand. / The hotshot’s clamor fills the air.
1954		Kerouac letter 19 Feb. in Charters I (1995) 407: ‘He’ the Conductor who made me ride outside the crummy.
1960, 1975		Wentworth & Flexner DAS.
1988		J.P. Spradley You Owe Yourself a Drunk 116: ‘Crummies,’ or cabooses, are often kept heated for those who travel in them.
1991		(con. 1920s) O.D. Brooks Legs 17: ‘What’s the crummy?’ I asked. ‘That’s the caboose.’.

2. (US Und.) a local jail, police station or workhouse.

1950

Goldin et al. DAUL 53/2: Crummy, n. [...] 2. A local lock-up, police station, or workhouse. ‘Thirty days in that crummy is worse than a treyer (three years) in the big house (state prison).’.

3. see crumb n.³