Railways are always built to stand above surrounding terrain to prevent track flooding, erosion of the bed and decay of the ties. In hilly and mountainous terrain, to avoid large slopes, the railway is at some places elevated, on an embankment or bridge/viaduct, and at some places in a cutting (ditch/trench) or tunnel. The same are also used for non-level crossings. In the case of many crossings, such as in a city, a longer stretch may be elevated or underground.