Green’s Dictionary of Slang

third rail n.1

[like the subway’s electrified third rail, such liquor ‘gives you a jolt’]

(US) extremely strong liquor; also attrib.

[US]O.O. McIntyre New York Day by Day 25 Nov. [synd. col.] [On the Bowery] they can live cheaply and and buy five-cent third-rail whiskey.
[US]O.O. McIntyre New York Day by Day 22 Mar. [synd. col.] Even on the Bowery a shot of ‘third rail’ whiskey costs 30 cents.
[US]O.O. McIntyre New York Day by Day 24 Aug. [synd. col.] A place [...] near Chatham Square where third rail whiskey was a nickel a shock.
[US]N. Klein ‘Hobo Lingo’ in AS I:12 653: Third-rail — strong wine.
[US]J. Callahan Man’s Grim Justice 4: He was always after my mother for the price of [...] a shot of third-rail booze.
Pittsburgh Post-Gaz. (PA) 22 Mar. 17/1: You take the case of one former boxer who when he gets squiffed on home brew or Third Rail varnish is not allowed to talk like the ordinary fellow who gets squiffed because if he does the boys tap their domes [...] and recall the day that Charley White tapped him on the koko.
[US]Goldin et al. DAUL 221/2: Third-rail. 1. Strong raw whiskey.
[US]Ragen & Finston World’s Toughest Prison 821: third rail – Strong, cheap liquor.