To repeat a post that I posted in another thread here pertaining to the same subject:
I, personally, think that opening the S&D being reopened throughout is unrealistic due to much of the formation having been lost to housing/industrial estates, roads, bypasses, and the removal of numerous bridges/viaducts. Certainly, the Bath end has lost Green Park Station to Sainsburys, the Midland Bridge was demolished, Dartmouth Avenue bridge demolished and Monksdale Road bridge demolished.
Some interesting pictures of some of the area in the gallery at this link:
http://www.twotunnels.org.uk
Furthermore, as per the link/website, the former trackbed through Bath to the Devonshire and Combe Down tunnels is to be converted into a cycle path.
Saying that, it would be fantastic if they could somehow link the parts of the line that have or are being reopened.
Although, as mentioned, the Avon Valley Railway could potentially find it's way back to the outskirts of Bath eventually. In fact, it's probably fair to say that the potential for reopening sections of the old Midland Bath - Mangotsfield route is considerably greater, since the trackbed is largely unaltered (save the fact it's a cycleway) and most of the stations are intact. Though it may be wishful thinking, I love the idea of seeing Mangotsfield with a train in one of it's platforms again!