A reinforced intermodal container has a corrugated steel wall with an outer side and an inner side. In a length direction the corrugations have alternating inboard and outboard flat panels with adjacent panels linked by web portions. A strengthening structure is welded to the container wall which provides flanges extending generally perpendicularly to the general plane of the wall. The flanges extend between plates welded to the inside faces of the outboard panels and flat strengthening plates welded to the inside faces of the inboard panels. The strengthening structure does not alter the outside width of the container in comparison with the outside width of the unreinforced container. The strengthening structure reduces the inside volume of the container by only a small amount compared with the volume of the unreinforced container.