A rail road car such as a double tub coal gondola car has a center sill that is narrow in the middle between the two tubs, and wider at the draft arms. The center sill has a transition section that provides a smooth, gradual transition between the respective bottom flange portions of the two without sharp changes in direction in the stress fields in tension and compression, and with little or no out of plane eccentricities in load path, and with a tendency to minimize or avoid local stress concentrations. The center sill includes a set of access apertures and corresponding bridges or ties that maintain the lateral halves of the center sill flange main portion in appropriate relationship to each other with respect to resistance to buckling. The tubs fasten mechanically to the sides of the center sill.