A road–rail vehicle is a self-propelled vehicle that can be legally used on both roads and rails. Combining the words "highway" and "rail", one is often referred to as a hi-rail truck or just hi-rail, sometimes spelled high-rail, HiRail or Hy-rail. They are normally converted rubber-tired road vehicles that have additional flanged steel wheels for running on rails. Because only one set of wheels would logically be used at a time, a pneumatic or hydraulic system is employed to lower and raise the rail wheels.