To simplify construction, reduce weight and improve mechanical properties, a sandwich panel substitution process is performed on corrugated sheets in the floor and roof of a rail vehicle car body. A requirement based selection is used to design the sandwich panels with the corrugated sheet mechanical characteristics as boundaries. Car body stiffness is evaluated by modal analysis. The derived panels reduce the mass of the car body by 600–700 kg. Results show the varying importance of the longitudinal, transverse and shear properties of the floor and roof panels, as well as how efficient the corrugated sheets actually are.