Green’s Dictionary of Slang

(US Und.) a large amount; a long prison sentence.

[1928		(con. WWI) H.F. Cruikshank ‘So This Is Flanders!’ Battle Stories July 🌐 I didn’t see anythin’ hum’rous about my number; it wasn’t one of them draft, boxcar numbers].
1932		V.F. Nelson Prison Days and Nights 23: These guys don’t talk any numbers that I can get my dukes on; all they talk is box-car numbers.
1950		Goldin et al. DAUL 33/1: Box-car numbers. Any large number; a long prison sentence.
1952		C. Hamilton Men of the Und. 320: Box-car number, Any large amount.
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 16: Boxcar-number – a large number.