I have seen cross-shaped turntables, albiet carrying only a single gauge (either 500mm or 600mm.) They were placed in the ‘main stem’ that ran along the fronts of a series of workshops (or in front of the only workshop) at several coal mines I ‘minefanned’ half a century ago. If a tramcar had to be moved into or out of a shop, the turntable (a circular steel plate with a 90-degree crossing welded to its top surface) only needed to be turned 1/4 turn once, not returned to its original orientation.
Chuck (Modeling Central Japan in September, 1964 - with mine trams and cross-turntables)
I’m not sure I see how it would make a dual-gauge turnout any easier. It looks like it moves the third rail from one side of the track to the other. There are places that could be helpful in building a large dual-gauge facility.
Indexing is excalty the same for both standard gauge and narrow gauge with a 4-rail turntable. I’ve done a coupe of dual-gauge projects for folks recently and the turntables are 4-rail. We just used separate narrow-gauge and standard gauge approach tracks. That avoids the complications of the transfer switch outlined in John Armstrong’s article. Roundhouse and whisker tracks may be either standard gauge, narrow gauge, or 4-rail dual-gauge. Easy.
Real railroad turntables are generally bridges, not disks. In any case, they wouldn’t bother with the complexity of frogs and a crossing in the center of a turntable with heavy engines being handled. That’s why 4-rail seems to be the usual choice.
The Burlington had a dual gauge turntable in Denver. Its standard and narrow gauge divisions shared it. There are plenty of pictures of two C&Sng 2-8-0s sitting on it. In the early days of the Colorado Central (predecessor to the Georgetown Loop and the C&Sng’s Clear Creek line), it had a dual gauge turntable at Golden, Co. I suspect that there were a number of others around.
The roundhouse in Golden did not have 3-rail stalls…the left half was ng only and the right half was std gauge. I suspect that the drop pits between the rails were the reason for this…as well as the lack of need to make each stall compatible with all the locomotives on the roster.
In support of using 4 rail dual gauge turntables, versus others with mechanical devices, is reliability. Remember that every loco in the engine house is held hostage to the proper functioning of the turntable. Railroad maintenance people would rather pay for additional rail, that requires little or no maintenance, rather than some sort of switch mechanism that constantly requires oversight. There is a famous photo of the SP roundhousein Roseville, completely filled with cab-forwards, that was able to be taken, because of a turntable malfunction. All those locos per parked, unable to make money for the company.