A railway car with a pair of improved sidewalls is provided. For some applications, the railway car may include a steel railway car underframe with an aluminum body mounted thereon. The car body preferably includes a pair of sidewalls and a pair of end walls. The car body may have an open top or a roof mounted thereon. Relatively, thin side posts are disposed on the exterior of the sidewalls to allow increasing cargo carrying capacity of the railway car. At least one longitudinal stiffener is disposed along the interior surface of each sidewall to provide sufficient strength for the sidewall based on anticipated loading while at the same time allowing reducing the number and the thickness of the side posts. For some applications the side posts and the longitudinal stiffeners may be formed by an extrusion process with substantially the same cross section.