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.
[US]Irwin Amer. Tramp and Und. Sl. 188: Third Rail.– Strong, cheap liquor, stuff which seems to galvanize the drinker much as would touching a ‘third’ or electrified rail.
[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.