Hello John,
as no one has replied to your request maybe I can help. This is my first posting on this site so be gentle with me!! Many years ago, probably in the 80's I went to this location with my cousin that had a small model steam train, we did not run the train but he showed me the location of the track.
From memory here goes. The farm is I think Bridge Farm, Stoke Canon, Exeter. EX5 4EE and I think the farm was owned by Mr. Horrell. Interesting if you Google Horrells Cider Steam truck you will find nice pictures of a steam driven vehicle. The location of the farm is as follows, from the centre of Stoke Canon drive toward Exeter on the A396. On leaving the village you cross a narrow section of road that includes river bridges with stone walls, at the end of this narrow section the A396 turns sharp right, take the small road/lane that leads straight ahead and Bridge farm is immediately on the left. From what I can remember you drove around to the rear of some farm buildings and the rail track was there. It seemed to run around the outside of a "slurry" lagoon so had strong country smells in the hot summer.
Hope this has been of use and "jogged" your memory, but feel free to ask if I can help any more.
Kind regards
Trevor Johnson
Exeter