Green’s Dictionary of Slang

1. (US prison) pertaining to the main prison population (and the rules to which it is subjected).

1933		‘Goat’ Laven Rough Stuff 181: They get their mates to furnish them with work [...] where they can get some food which is not the ‘main line chuck’ (regulation prison food).
1935		D. Lamson We Who Are About to Die 92: Mainline chow is another matter.
1967		M. Braly On the Yard (2002) 251: No reason for you to eat mainline [...] You never ate mainline in Tracy, did you?
1999		E. Bunker Mr Blue 151: The condemned, who were far fewer and more quickly executed back then, were fed much better than the mainline convicts.

2. (orig. US drugs) pertaining to narcotics use and addiction.

1956		H. Gold Man Who Was Not With It (1965) 278: He didn’t have to sell mainline sugar in addition.
1961		T. Taylor Baron’s Court All Change (2011) 71: [P]rodding his bruised arm [...] desperately searching for a vein to send the poison in to a mainline station.
1961		‘Lou Rand’ Gay Detective (2003) 79: You can get anything in that joint from a piece of trade to a main line shot.
1972		E. Grogan Ringolevio 40: He had given himself his wings – his first mainline fix.