Main Entry: Rail Function: noun Etymology: Middle English raile, from Middle French reille ruler, bar, from Latin regula ruler, from regere to keep straight, direct, rule – more at RIGHT 1 a : a bar extending from one post or support to another and serving as a guard or barrier b : a structural member or support 2 a : RAILING 1 b : a light structure serving as a guard at the outer edge of a ship's deck c : a fence bounding a racetrack 3 a : a bar of rolled steel forming a track for wheeled...