Green’s Dictionary of Slang

1. (also Johnny O’Brien) a freight train or one of its boxcars; a side-door Pullman.

1902		Wash. Times (DC) 14 Sept. 10/4: John O’Brien— A freight train.
1909		F.H. Tillotson How I Became a Detective 92: John O’Brien – A freight train; a box car.
1914		Jackson & Hellyer Vocab. Criminal Sl. 50: john o’brien [...] A freight train, used in contradistinction to a ‘rattler’.
1921		P. & T. Casey Gay-cat 61: I’ve bin wandering here and there, on the rods and blinds and in John O’Briens.
1931		Irwin Amer. Tramp and Und. Sl. 111: John O’Brien. – A slow or ordinary freight train, as against the fast freight.
1949		Monteleone Criminal Sl. (rev. edn) 134: johnny o’brien A box car.
1952		C. Hamilton Men of the Und. 323: John O’Brien, A box-car.

2. an empty safe.

1914

Jackson & Hellyer Vocab. Criminal Sl. 50: john o’brien [...] a moneylesss safe.

3. a hand-car.

1926

J. Black You Can’t Win (2000) 110: Johnnie and Sanc had [...] strangled any suspicion that might fall on them by taking the ‘John O’Brien’ — the bums’ term for hand car, so called because every other section boss in those days was named O’Brien.