A road–rail vehicle is a vehicle which can operate both on rail tracks and a conventional road. They are also called hi-rail, from highway and railway, or variations such as high-rail, HiRail, Hy-rail, etc.

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; the rail wheels are raised and lowered as needed. Purpose-built road–rail vehicles also exist.

Trucks running on railway tracks : VIDEO

Such vehicles are normally used for railroad right-of-way maintenance during engineering possessions of the line. They 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 (the line is closed to normal traffic).

They are generally designed to be insulated, thus they do not activate track (signaling) circuits, although some rail operators, normally those operating remote lines without boom gates, etc., prefer them to be non-insulated so that they are detectable by train safety systems.

Self-propelled maintenance vehicles for maintenance of the track and for shunting wagons are much more convenient to use if they can transfer to the road to reposition or otherwise get out of the way. Because relatively light loads are involved, the problems plaguing the Road Transferable Locomotive are avoided.

Road–rail vehicles, particularly those used for inspection purposes, have been involved in a number of serious incidents, including deaths. There has been ongoing discussion regarding maintenance and inspection standards, including load and load distribution, to minimise the risk of failures.

Factors leading to derailment include failed locking equipment, wheel failure, damaged rail wheel support systems, inappropriate tires, and uneven or overloading issues.

When operating in road-going mode, drivers have to remember that the dynamics of the vehicle will be changed due to the increased weight at the front and rear of the chassis. Some manufacturers have developed systems that allow the rail wheels to be stored almost entirely inside the original bodywork[1] thus moving the centre of mass closer to the road axles. This greatly improves the on-road driving performance of the vehicle.