A turntable is a large circular platform which is used to turn locomotives and other rolling stock for railways. A well-engineered turntable is designed in such a way that the efforts of only two or three people are needed to operate the turntable, even when dealing with very large and heavy locomotives. A related railroad term is “wye,” a term used to describe a special configuration of track which essentially allows trains to make really big three point turns so that they can turn to face the direction they came from.
The reason the turntable was developed was because early steam locomotives were somewhat difficult to run in reverse. In some cases, locomotives lacked a reverse gear altogether, and in other instances, the reverse speed was very slow and clumsy, which could be a liability at a busy switching station or hub. As a result, train companies started building turntables so that they could quickly turn their locomotives around.
To use a turntable, a locomotive or railway car is driven or pushed onto the platform and locked in place before the platform is spun to face in a new direction. In addition to being used to turn rolling stock 180 degrees, turntables can also be used to shunt locomotives off onto sections of track positioned at a variety of angles relative to the turntable. Once the platform has been turned and locked in place, the locomotive can be unlocked and moved.
Most modern locomotives have a reverse gear, or they are capable of being run in either direction, making them very easy to move around. However, turntables are still in use at railway companies where steam locomotives are used, and at sites where room to maneuver is limited, encouraging the use of a space-efficient turntable to move locomotives around.
Wyes, on the other hand, are made by creating a roughly triangular set of tracks, allowing trains to move back and forth along the arms of the wye to change direction. In addition to being used to turn a train around, a wye can also allow a train to move out of the way to yield to another train on the same track.
In addition to being used for locomotives, a turntable can also be used for cable and street cars, in which case the car's gripping mechanism must be detached before it is rolled onto the turntable. For people who want to see a turntable in action, a well-known cable car turnaround can be found in active use at Market and Powell Streets in San Francisco, California; turntables can also be seen at railway museums.