Subassemblies for railroad tank cars are herein disclosed. In some embodiments, the subassembly includes a center sill including a generally horizontal top member and a pair of upright side plates spaced apart laterally from each other and a head brace having a lower margin welded to the top member of the center sill and an upper margin welded to the head of the tank. Each of the side plates having a top surface and an opposed bottom surface, the top surface being planar within a longitudinal central portion and at least a substantial part of a first longitudinal end portion. The bottom surface is planar and parallel to the top surface within the longitudinal central portion and a second longitudinal end portion. At least a substantial part of the bottom surface within the first longitudinal end portion has a shape that includes or approximates one or more radiuses of curvature.