Green’s Dictionary of Slang

1. the point of twelve in craps dice; cite 1921 is a misreading.

1909		T.A. Dorgan in Zwilling TAD Lex. (1993) 19: Coffroth poured out a pair of sixes (box cars).
1919		A. Baer Two and Three 11 Jan. [synd. col.] Even the Gobb of Siam knows that two boxcars are a pair of sixes.
1921		Truth (Brisbane) 13 Nov. 9/4: In the language of the [craps] players [...] nine is a ‘box-car’.
1927		Bridgeport (Conn.) Telegram 21 Nov. 4/5: De dice am clickin’ so fast you’d think you was in a telegraph office, and Nappy makes ebery point on de dice and some dat wasn’t on ’em – Little Joe, Feber in de South, Five and a Half, Sixty Days, Two Months and a Half, Eighter From Decatur, Eight and Three Quarters, Three Boxcars and the Caboose.
1937		Wash. Post 3 Oct. B8/6: A ten is ‘Big Dick from Dixie,’ and a twelve is ‘Boxcars,’ but three is just plain ‘Craps.’.
1949		Monteleone Criminal Sl. (rev. edn).
1957		C. Himes Imabelle(1969) 23: Abie-the-Jew bet the dice to win or lose, barring box cars and snake-eyes.
1959		T.C. Schelling in Rev. of Economics and Statistics 3 Oct. 214/1: There are other ways, such as having the referee roll dice every few minutes, calling off the game whenever he rolls boxcars.
1965		C. Himes Rage in Harlem(1969) 23: [as 1957].
1972		E. Grogan Ringolevio 232: Double sixes, boxcars.
2001		Word for the Wise 31 Aug. [US radio script] The slang boxcars comes from the resemblance of a pair of sixes to roofed freight cars.

2. thus fig., bad luck.

1933		N. West ‘Miss Lonelyhearts’ in Coll. Works (1975) 249: Get tanked, grab what’s on the buffet, use the girl upstairs, but remember, when you throw box cars, take the curtain like a dead game sport, don’t squawk.
1937		E. Anderson Thieves Like Us (1999) 7: Boxcars won’t jump up in your face every throw.
1950		Goldin et al. DAUL 33/1: Boxcars. A stroke of misfortune. [Ibid.] Throw boxcars. (Chiefly rural; Central Western and Southwestern States) To suffer a stroke of misfortune; to run into hard luck.
1972		E. Grogan Ringolevio 398: I’d still be throwin’ those same boxcars and crappin’ out on history.