Green’s Dictionary of Slang

1. a racetrack fan who stands next to the rails to get as near as possible to the racing.

1891		N.Y. Mercury Dec. in Ware (1909) 206/1: The ‘rail-birds’, as certain people are called who closely watch the work of horses on the race tracks, would do well to keep an eye on Tommy Ryan.
1903		‘Hugh McHugh’ Out for the Coin 87: In the person of the railbird next to me I discovered a friend, old uncle Henry Carroll.
1929		D.H. Clarke In the reign of Rothstein 92: His fast trials had been hidden so well, that even the rail birds and cockers knew nothing good about him.
1933		W.R. Burnett Dark Hazard (1934) 44: Rail-birds sitting on the fences of the back stretch, gamblers trying to keep the odds up.
1944		Chicago Daily News 4 Aug. 24/1: The Homewood rail birds, captivated by her unusually lengthy strides, decided to measure her prints [DA].

2. a fan or spectator who crowds round the rails that surround a big game in a casino, or other place of entertainment.

1981		(con. 1930s) O’Day & Eells High Times Hard Times 36: LeNac cut out with this red-headed railbird he’d been romancing.
1985		E. Leonard Glitz 125: Could be a railbird, waiting to grab a few chips.
1991		P. Asch Racetrack Betting 27: No one can be completely sure— not us, not you, and not the railbird who assures you with absolute certainty that ‘his’ jockey has just ‘thrown’ the last race.