The railway turntable is constructed for changing the direction of the rail engine. Triangle and turntable are used for the same purposes but the cost of construction is different. Turntables installation are more costly than a triangle track junction. But it is possible to construct at important stations where space available is limited. Railway turntable is circular in shape and installed in a circular pit.
A turntable consists of a track on a level platform which is supported by a pair of girders. the girders are fixed on a central pivot. There are two or more tracks radiate from the circumference of the pit circle.
When the direction of an engine is to be changed, the engine is brought on the turntable, and after loosening the locking keys, the turntable is rotated with a certain mechanism. After a rotation of 180 degrees, it is then placed on the same tracks or on any other track with exact rotation as needed.
The diameter of the turntable depends on the length of the engine. Hence, the diameter should be long enough to accommodate the longest engine feasible to use the turntable. In India, usually, the length of the turntable is 19.8 metre, but some turntables are of 25.91 metre length also.
Video of Railway Turntable