When I first saw your turntable idea I was pretty sure the Talylln had a device like this that fitted under a rail-car to turn it round.
I have found a picture of the vehicle in a book called “North Wales Branch Line Album”. The picture shows the mercury tractor (TR no 7) pulling a train of empty wagons. The caption says:
The mercury tractor was of uncertain habits and went backwards like a tortoise and a centre jack for turning it round was installed underneath. The photograph was taken in near quarry siding in 1954.
I am sorry I don't have a scanner so I can't post the picture.
Peter, do what I do without a scanner. I take a photo of the photo and then if needed, I run it through Photoshop and "clean" it up and then I can post it here. Much easier than messing with a scanner.
This was indeed Talyllyn No 7. A not very successful conversion of an old Mercury tractor to rail use - it only lasted about 4 years as such. The 'jack' was a portable jack placed beneath this thing when it required turning; not like a built in RV type levelling jack. Apparently it needed at least 2 men to ensure the correct balance whilst turning. I don't know where the jack was carried.
I have found another photo if you require it - intriguingly with a different wheel arrangement. My camera de-materialised from my suitcase during Carnival-Cunard's baggage handling operation i.e. some bastard stole it & I don't have a scanner. I got Mrs Salada to photo the print for you with her Eyepad thingy but I can't move the photo she sent to my E mail inbox over to my desk-top. If you need a photo of No 7 then PM me & please wait till I've got a new camera.