A viaduct is a specific type of bridge that consists of a series of arches, piers, or columns supporting a long elevated railway or road. Typically, a viaduct connects two points of roughly equal elevation, allowing direct overpass across a wide valley, road, river, or other low-lying terrain features and obstacles. The term viaduct is derived from the Latin via meaning "road", and ducere meaning "to lead". Viaducts are commonly used in many cities that are railroad hubs, such as Chicago, Birmingham, London, and Manchester. These viaducts cross the large railroad yards that are needed for freight trains there, and also cross the multi-track railroad lines that are needed for heavy rail traffic. Viaducts mainly connect two points of the terrain which are similar in height in order to carry mostly rail and road traffic. They are built over gorges, canyons, valleys, and sometimes water. Unlike more complex and diversified construction of bridges, viaducts consist of the main carrying surface which is supported by arches of equal spans mostly built of stone or concrete.