I did an offical tour of Monkton Farleigh when it was a museum in the early eighties.
An awe inspiring place, absoloutely huge.
There have always been rumours that all of the old stone quarries / ammo dumps / bunkers in the area were linked by rail, but before, during and after the museum at MF was open, they have had problems with intruders.
It has been very well explored over the years, and there are photos on many websites, but I've never heard of anyone finding anything that could have been a (main line) rail link down there. There were narrow gauge tracks everywhere, but I don't think the SSR could have been.
Most of the site is now owned by Wansdyke security who use it for secure storage so I don't think "visits" are possible any more. :-(