Railway turntables are one of the less-covered components of the classic rail era. In the steam engine’s days of glory, they were heavily used to turn(change the moving direction/face of the) locomotives.
Two years ago, I got a chance to view(in-action) a hand-driven, nano-turntable. Near the end of the video, note the operator unlock the turntable. The locking was to prevent undesired rotation and to align the bridge rails with the exit track.
Contrary to the popular belief, they are still used for the modern freight trains where space constraints make it impossible to build a wye. It should be noted that these turntables were/are an energy-efficient, ‘green’, low-cost option to turn locomotives.
Life is about being kinetic 