Green’s Dictionary of Slang

snail n.

1. (Aus.) a shepherd, a musterer, one who mends boundary fences [? the speed of his progress].

[Aus]Worker (Brisbane) 4 Sept. 8/4: By ‘lizards’ he means musterers, sometimes he calls them ‘snails’.
[Aus]Baker Aus. Lang. 63: Shepherds have been known variously as lizards, crawlers, snails and motherers.

2. (US) a freight train [its (lack of) speed].

[US]Daily Trib. (Bismarck, N.D.) 23 Oct. 4/1: An express train is a ‘flyer,’ a freight train a ‘snail.’.
D. Burley N.Y. Amsterdam Star-News 20 Sept. 15: They pop in on the snail with many twirlers like rubbernecks.

3. (US tramp) a cinnamon roll or bun [? its shape].

[US]Wathena Times (KS) 28 Aug. 3/1: Try Stephan’s coffee cake and snails.
[US]El Paso Herald (TX) 9 Feb. 10/4: ‘Have you tried our celebrated Danish Coffee Snails>’.
[US]F. Williams Hop-Heads 62: I went into the dairy lunch, ordered a cup of coffee and a ‘snail’.
[US]G. & S. Lorimer Stag Line 166: ‘Nothing in sight but these sinkers and snails. Whole plate of ’em.’ [...] ‘These snails are pretty tasty.’.
[US] ‘Argot of the Sea’ in AS XV:4 Dec. 450/2: coffee an, coffee and. Coffee and sinkers (doughnuts) or coffee and snails (cinnamon rolls).
Quad-City Times (Davenport, IA) 20 Sept. 12/1: he saw ‘coffee and snails’ on the menu.
Times (San Mateo, CA) 4 Sept. 11/8: Turnbull [...] served coffee and snails to teachers and parents.
[US]Ragen & Finston World’s Toughest Prison 818: snails – Cinnamon rolls.
[US]Sheboygan Press (WI) 9 Feb. 16/4: Root’s Monday Morning Koffee Klatch [...] Doughtnuts [...] Snails.

In compounds

snail-eater (n.)

(US) a French person.

A. Baer Says ‘Bugs’ Baer 16 Oct. [synd. col.] A brother ‘against’ brother act between the snail-eater [i.e. Carpentier] and Jack Dempsey.
Sphere (London) 27 Sept. 32/1: My! What manners that Snail Eater had — smooth as talcum powder.
snail shaker (n.)

(US) a gambler (? at dice).

[US]Western Police Gaz. (Cincinnati, OH) 29 Mar. n.p.: Terrible Shaking Among the Gamblers [...] the snail shakers did not get wind of what was brewing until they found themselves in the hands of the police.