A railroad box car has a combined underframe and floor structure that includes a floor sheet having an upper surface upon which to carry lading, and an underside to which a center sill, cross-members, and floor stringers are welded. The floor sheet defines the upper flange of the center sill, the cross-members, and the stringers, such that the center sill, cross-members, and stringers are mutually flush. For assembly, the floor sheet is upside-down, and the center sill webs, stringers, and tie-down mounting rails to it with longitudinal welds. The cross-members and center sill web separators are positioned cross-wise and welded in place. The floor sheet is a non-nailable surface that caps the underframe structure. The stringers are channels welded toes-in to the underside of the floor sheet with the spacing between the center sill web, stringer toes, and edge channel rails being roughly equal.