I've found this kind of information quite hard to pin down for the larger scale turntables we're designing - there are so many different types - light, heavy, wide, narrow, shallow, deep, and some downright agricultural. The best thing is to get a photo of the particular type that interests you and try scaling it. I see you are in Aberdeenshire, so if you haven't already tried it, it might be worth going over to Aviemore to take some dimensions off the working turntable there. It might be a bit bigger than 54 ft, but it should be a good guide to the general proportions. It's relatively modern (post war) and I think it came from Kyle of Lochalsh, so it would have been sized to turn a Black 5. Also, I wonder if the GNoSR turntable pit is still there below Union Terrace Gardens in Aberdeen? You could still see the remains of it last time I was there, and there might be enough left to give you some ideas.
Maybe you can let us know how you get on with it. The 4mm scale manufacturers only seem to like the largest turntables, so those of us who want the medium ones have to build our own.
A feature of some Ransomes and Rapier and Cowans Sheldon turntables supplied to the GNR was a tapered bridge with a matching conical well profile. A good photo of the Hornsey 52' table shows the side walls to be about 2' high, with the centre of the well at least 6' deep, as an example. A similar arrangement applied to the Hatfield 40' table, although less of the depth of the well may be seen. Whether this was general practise, or a peculiarity that the GN specified I have no idea.
The Swanage turntable is about the size you are looking for, the well is 2ft 6in deep and has a raised section round the edge with the rail support to the table on it. There are a lot of photos available on the swanage railway web site. I made my O gauge one from scaling of the photos using known dims.
Back from travels, so I can offer a brief follow-up to my last note. I just checked the Highland Railway turntable list in Cormack and Stevenson's "Highland Railway Locomotives, Book 1" (RCTS). The Aviemore turntable is listed as being 55ft, and the last one at Kyle was 54ft, so you might find some material if you are able to get to Aviemore to take a few dimensions. There's a photo of it at http://jonathonag.fotopic.net/p13158020.html though the bridge itself looks a bit too modern for what you need. Early turntables often had a wooden planked floor at rail level, supported from the bridge by cantilever beams and completely obscuring the pit.
Hawkins and Reeve's series of books on engine sheds are an excellent resource and have a good selection of photos of later installations. A bit farther afield, there's good book called "Engine Houses and Turntables on Canadian Railways, 1850-1950" by E.F. Bush, published by Boston Mills. It's well illustrated, and the older turntables shown in it have proportions that would probably be applicable. (I think it is sold in the UK, so it may be available through inter library loan). It's good for mechanical detail, too.
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