The rail is the sideline at a poker table - the (often imaginary) rail separating spectators from the field of play. Watching from the rail means watching a poker game as a spectator. People on the rail are sometimes called railbirds. "Going to the rail" usually means "Losing all one's money".
RAIL
Meaning of RAIL in English
Poker English dictionary. Английский словарь покера. 2012