Rail transport is a means of conveyance of passengers and goods, by way of wheeled vehicles running on rails. It is also commonly referred to as train transport. In contrast to road transport, where vehicles run on a prepared flat surface, rail vehicles are also directionally guided by the tracks on which they run. Track usually consists of steel rails installed on ties (sleepers) and ballast, on which the rolling stock, usually fitted with metal wheels, moves. However, other variations are also possible, such as slab track where the rails are fastened to a concrete foundation resting on a prepared subsurface.