Overview
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 convert to rail vehicle for the final journey to the worksite. This avoids the complex maneuvers that would be associated with a road vehicle accessing the worksite if the worksite is not near a road. Since they are normally converted road vehicles, they would not fare well in a collision with a heavy rolling stock and therefore can only drive on rail tracks under an engineering possession. They are generally designed to be insulated, thus they do not activate track (signaling) circuits.
Read more about this topic: Road-rail Vehicle