Abstract: An enclosed trailer for towing by an over the road truck is provided having increased internal width for a given external width. The side walls of the trailer are constructed from a series of vertically extending, spaced apart, rigid plates, preferably metallic plates, and metallic reinforcing members which have an inwardly projecting, vertically extending rib that is positioned between the adjacent edges of adjacent pairs of plates. The reinforcing members are riveted or otherwise rigidly secured to the outside of a pair of plates. Furthermore, the plates and reinforcing members are also rigidly secured to vertically extending flanges of the longitudinally extending top and bottom beams which form the juncture between the side walls and the top wall, and the side walls and the bottom wall, respectively. Because all elements of the trailer side walls are rigidly joined to the top and bottom beams, the overall strength and rigidity of the trailer is increased.