Railbird - Rail
1) Literally "the rail" refers to a railing separating the gaming tables from spectators
2) Railbird: a non-player watching games
3) Railbirds: the group of broke or near-broke sometimes-players who seem to do nothing but hang around casino cardrooms
4) On the rail: could refer to railbirds, or a player recently eliminated from a tournament, or a player on a waiting list to get into a game