All rails are spaced 9 mm apart but the height can differ. Rail height (in thousandths of an inch) is expressed as a "code": thus, Code 55 rails are high while Code 80 rails have a height of . Common real railroad rails are at least tall and can be taller on some roads, so at true scale the rails would be about high. Many older N-scale models may not run well on Code 55 track as their flanges are often unrealistically large, causing the wheels to bounce along the ties instead of ride along the railhead. Wheelsets with these large flanges are colloquially known as 'pizza cutters' due to a resemblance to the kitchen utensil.