Turntable (railroad)

In rail terminology, a railway turntable or wheelhouse is a device for turning railroad , 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 weigh against the construction of a turnaround wye. In the case of steam locomotives, railroads 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 consist, the locomotives can be arranged so that the consist can be operated "front end first" no matter which direction the consist is pointed. Turntables were also used to turn observation cars so that their windowed lounge ends faced toward the rear of the train.

Early wagonways were industrial railways for transporting goods—initially bulky and heavy items, particularly mined stone, ores and coal—from one point to another, most often to a dockside to be loaded onto ships. These early wagonways used a single point-to-point track, and when operators had to move a truck to another wagonway, 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 turnplates or sliding rails. Turnplates 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 wagonway, and they swung around a central pivot. Loaded wagons could be moved onto the turnplate, and rotating the turnplate 90 degrees allowed the loaded wagon to be moved to another piece of wagonway. Thus, wagon weight was limited only by the strength of the wood used in the turnplates or sliding rails. When iron and later steel replaced stone and wood, weight capacity rose again.

...
Wikipedia