Concept
A road–rail vehicle or a rail–road vehicle is a dual-mode vehicle which can operate both on rail tracks and roads. They are also known as two-way vehicles (Zweiwegefahrzeug), hi-rail (from highway and railway, or variations such as high-rail, HiRail, Hy-rail), and rail and road vehicles. They are often converted road vehicles, keeping their normal wheels with rubber tires, but fitted with additional flanged steel wheels for running on rails. Propulsion is typically through the conventional tires, the flanged wheels being free-rolling, used to keep the vehicle on the rails; the rail wheels are raised and lowered as needed. There are also purpose-built road–rail vehicles. In case of jeep trains, road wheels are directly replaced with railway wheels. Vehicles with tires need special areas like level crossings to change modes. A vehicle on caterpillar tracks rather than road wheels, which allows mode change anywhere without the need to use a level crossing, has been proposed and modelled by Chinese engineers. Most of such vehicles are used for railroad right-of-way maintenance during engineering possessions of the line, when a section of the line is handed over for maintenance and operational trains are blocked from entering the section. Military used railroad vehicles take advantage of intact railways for locomotion or are used as emergency locomotives. The usage as passenger buses is rare and mostly experimental. Maintenance railroad vehicles can be driven on roads to near the site and then converted to a rail vehicle for the final journey to the worksite. This avoids the complex maneuvers that would be associated with a road vehicle accessing a worksite that is not near a road. Since they are normally converted road vehicles, they would not fare well in a collision with heavy rolling stock and therefore can normally only drive on rail tracks under an engineering possession, when the line is closed to normal traffic. They are generally designed to be insulated, thus they do not activate railway signalling circuits.
