noun Informal.

a horse-racing fan who watches races or workouts from the railing along the track.
any kibitzer or self-styled critic or expert.

Origin of railbird

1890–95, Americanism; rail1 + bird in sense “frequenter,” as in jailbird, yardbird
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019