Abstract: There is provided, in a preferred embodiment, an end cap for the rails of an extension plank or the like in the form of a molded one-piece open box which fits over the ends of a set of rails, with the ends of the rails being fastened together therein in spaced apart relation by means of rivets or other fasteners extending through holes which may be formed in the end cap during the molding process. The end cap is preferably formed of an injection-molded thermoplastic, such as an acrylonitrile-butadiene-styrene material. Manufacturing steps, such as grinding of sharp corners, cutting to length, and forming of holes, required by conventional end pieces are eliminated. Maximum-load safety warnings may be permanently molded into the end cap.