A rail yard, railway yard, railroad yard (US) or simply yard, is a series of tracks in a rail network for storing, sorting, or loading and unloading rail vehicles and locomotives. Yards have many tracks in parallel for keeping rolling stock or unused locomotives stored off the main line, so that they do not obstruct the flow of traffic. Cars or wagons are moved around by specially designed yard switchers (US) or shunters, a type of locomotive. Cars or wagons in a yard may be sorted by numerous categories, including railway company, loaded or unloaded, destination, car type, or whether they need repairs. Yards are normally built where there is a need to store rail vehicles while they are not being loaded or unloaded, or are waiting to be assembled into trains. Large yards may have a tower to control operations. Many yards are located at strategic points on a main line. Main-line yards are often composed of an up yard and a down yard, linked to the associated direction of travel. There are different types of yards, and different parts within a yard, depending on how they are built. Freight yards For freight cars, the overall yard layout is typically designed around a principal switching (US term) or shunting (UK) technique: A flat yard has no hump, and relies on locomotives for all car movements. A gravity yard is built on a natural slope and relies less on locomotives; generally locomotives will control a consist being sorted from uphill of the cars about to be sorted. They are decoupled and let to accelerate into the classification equipment lower down. A hump yard has a constructed hill, over which freight cars are shoved by yard locomotives, and then gravity is used to propel the cars to various sorting tracks; Sorting yard basics In the case of all classification or sorting yards, human intelligence plays a primary role in setting a strategy for the switching operations; the fewer times coupling operations need to be made and the less distance traveled, the faster the operation, the better the strategy and the sooner the newly configured consist can be joined to its outbound train. Switching yards, staging yards, or shunting yards are typically graded to be flat yards, where switch engines manually shuffle and maneuver cars from (a) train arrival tracks, to (b) to consist breakdown track, to (c) an consist assembly track,