Not surprisingly, early turntables were very small, sometimes just six feet in circumference since nothing larger was needed with the relatively small locomotives and cars in use during much of the 19th century. However, this changed as equipment grew larger, heavier, and longer thanks to new technologies like the air-brake, more advanced steam locomotives, and a greater demand for rail service. During the 20th century most roundhouse terminals featured turntables of at least 70 feet with some as large as 120 feet or more in circumference. According to the magazine Trains, most Class I railroads of the era found that turntables of between 80 and 100 feet often suited their needs. Additionally, with these larger tables manual "Armstrong" designs fell out of favor.
Powered tables usually featured either compressed air or pneumatics/hydraulics to turn them. Since turntables are no longer needed within today's industry you might not realize just how vital these devices once used to be. For instance, the above mentioned periodical, Trains, did a short piece from its January, 1945 issue which described how Baltimore & Ohio forces rebuilt its turntable in Chillicothe, Ohio where once a small terminal was located along its St. Louis main line. The railroad wished to make the table deeper and wider to support larger and heavier steam locomotives (although interestingly, by that time, steam was in steep decline as main line power on most lines). To make sure that the work did not severely interfere with passenger and freight train schedules it was timed down to the minute and all of the locomotives operating out of the terminal were moved outside the roundhouse to nearby ready tracks.
The actual project began at 7 a.m. and is said to have been completed in just 10 hours! The purpose of the devices, however, was mostly made obsolete by the diesel. This new type of locomotive could be operated just as efficiently bidirectionally, which eliminated the need for turning them as often as steamers. When they did need to be turned a common wye (literally, a "Y"-shaped track setup) would suffice. Without the need for a turntable railroads also did not require the accompanying roundhouse, although many remained in use for decades as maintenance facilities. Today, turntables still have found a way to hang on as a need to turn steam locomotives, and some diesels, on excursion railroads and museums. They not only offer a practical way to turn the units without the need for a wye but also give a chance to allow the public to see how these interesting devices worked when they were regularly in use.
If you are interested in fallen flags and classic systems before the modern merger movement took hold I would suggest purchasing one, or all three volumes of Mike Schafer’s Classic American Railroads series of books. The publications feature a wealth of information and are loaded with colored photographs covering names like the Baltimore & Ohio, Santa Fe, Pennsylvania, Milwaukee Road, and lots more. If you have any interest in most famous bygone railroads which once operated in the U.S. I would strongly recommend picking up one, two, or even all three of Mr. Schafer’s book. I truly cannot say enough just how enjoyable and fun they are to read and peruse through.