You can still see bits of it going down through the Collingbournes, the lines are (or were last year) still in situ at Ludgershall whee the army has a train depot. Of course going north the line is being preserved at Blunsdon Station, I think it now runs almost up to Mouldon Park and they do hope to get it as far as Cricklade in the other direction.
Up until about I suppose 7 or 8 years ago now, you could walk all the way to South Cerney on the old track bed...not sure what happened at Ciren as never tried it. The Swindon starting end picking up just beyond Mannington, where it got lost before the OT cycle path. The last time I did it though, whereas before the route was doable previously a fucking great housing estate had been dumped on it and the track bed ripped out
I was coming back from South Cerney.
Couple of years ago went up there to investigate the latest situation having read about the Moulden Hill thing and no evidence of a link.
I've explored the line south of Marlborough....last time I did it again a while back, the bit heading out of Marlborough, was completely overgrown, it was just about followable up to the cutting opposite Granham Farm, but at this point waded into 12 foot high scrub....in the spirit of adventure gave it my best shot, but without a machete impossible.
Marlborough, used to have 2 lines, and so 2 stations, Marlborough Low level, I think was on the M and SWJ, and Marlborough High Level was on the Pewsey main line branch. Confusingly the next stops, were Savernake High and Savernake Low respectively.