Rail1  —n. 1 level or sloping bar or series of bars: a used to hang things on. B as the top of banisters. C forming part of a fence or barrier as protection. 2 steel bar or continuous line of bars laid on the ground, usu. As a railway. 3 (often attrib.) Railway. —v. 1 furnish with a rail or rails. 2 (usu. Foll. By in, off) enclose with rails.  off the rails disorganized; out of order; deranged. [french reille from latin regula *rule]