encountered. With simple wagon derailments where the final position is close to the proper track location, it is usually possible to pull the derailed wheelsets back on to the track using rerailing ramps; these are metal blocks designed to fit over the rails and to provide a rising path back to the track. A locomotive is usually used to pull the wagon. A huge disadvantage of doing it this way is that the ramps can seriously damage the infrastructure because of which this procedure may not be used in several countries. If the derailed vehicle is further from the track, or its configuration (such as a high centre of gravity or a very short wheelbase) make the use of ramps impossible, jacks may be used. In its crudest form, the process involves lifting the vehicle frame and then allowing it to fall off the jack towards the track. This may need to be repeated. A more sophisticated process involves a controlled process using slewing jacks in addition. This combination of lifting and sliding is called a hydraulic rerailing system. A system consisting of high pressure hydraulic lifting jacks (used for lifting the train) so a sliding system can be positioned underneath the vehicle. The sliding system consist of a beam (also called a bridge) with sleds or carriages which are moved laterally with a horizontally positioned high pressure hydraulic jack to push the vehicle back above track. After which it is lowered again on the track. Photographs of early locomotives often indicate one or more jacks carried on the frame of the locomotive for the purpose, presumed to be a frequent occurrence. When more complex rerailing work is needed, various combinations of cable and pulley systems may be used, or the use of one or more rail-borne cranes to lift a locomotive bodily. In special cases road cranes are used, as these have greater lifting and reach capacity, if road access to the site is feasible. In extreme circumstances, a derailed vehicle in an awkward location may be scrapped and cut up on site, or simply abandoned as non-salvageable. Examples Note: there is a large list of railway accidents in general at Lists of rail accidents. Primary mechanical failure of a track component In the Hatfield rail crash in England in 2000, which killed four people, rolling contact fatigue had resulted in multiple gauge corner cracking in the surface; 300