 In rail terminology, the railway turntable or wheelhouse is a device for turning railway rolling stock, usually locomotives, so that they can be moved back in the direction from which they came. This is especially true in areas where economic considerations or a lack of sufficient space have served to wait against the construction of a turn-around Y. In the case of steam locomotives, railways needed a way to turn the locomotives around for return trips as their controls were often not configured for extended periods of running in reverse and in many locomotives the top speed was lower in reverse motion. In the case of diesel locomotives, though most can be operated in either direction, they are treated as having front ends and rear ends often determined by reference to the location of the crew cab. When operated as a single unit, the railway company often prefers, or requires, that a diesel locomotive be run front-end first. When operated as part of a multiple-unit locomotive consists, the locomotives can be arranged so that the consist can be operated front-end first no matter which direction the consist is pointed. Train tables were also used to turn observation cars so that their windowed lounge ends faced toward the rear of the train. Early wagon ice were industrial railways for transporting goods initially bulky and heavy items, particularly mined stone, doors and coal from one point to another, most often to a dockside to be loaded onto ships. These early wagon ice used a single point-to-point track, and when operators had to move a truck to another wagon ice, they did so by hand. The lack of switching technology seriously limited the weight of any loaded wagon combination. The first railway switches were in fact wagon turn plates or sliding rails. Turn plates were initially made of two or four pieces of wood, circular in form, that replicated the track running through them. Their diameter matched that of the wagons used on any given wagon ice, and they swung around a central pivot. Loaded wagons could be moved onto the turn plate, and rotating the turn plate 90 degrees allowed the loaded wagon to be moved to another piece of wagon ice. Thus, wagon weight was limited only by the strength of the wood used in the turn plates or sliding rails. When iron and later steel replaced stone and wood, weight capacity rose again.