The S&D was never a "main line" in the accepted sense of the word. Except for 10 summer Saturdays a year (or thereabouts) it had one express service per day. Leaving those 10 summer Saturdays to one side, the passenger service could easily be handled by 2P 4-4-0s with a three coach set. Freight traffic was predominantly coal from the North Somerset coalfield, the last pit of which closed in 1974.
To be as charitable as possible, indigenous traffic would be negligible.
Looking at Google Earth, all the way from Yate to Holes Bay junction, we find the following problems with reopening (E&OE

):
1. The A4174 ring road now occupies the trackbed between the spur to Shortwood Hill and Siston Common (1.65 miles)
2. Sustrans take a dim view of reopening railway formations that have been turned over to cycle paths - that puts a spanner in the works all the way from Siston common to Brassmill Lane
3. Warmley level crossing - the Highways authorities wouldn't think too highly of that reopening to rail traffic
4. Brassmill Lane to Green Park. The entire formation has been demolished/ redeveloped although the river bridge is still there just beyond Weston station.
5. Green Park station now put to other uses
6. Bridges taken out on the climb to Devonshire Tunnel
7. Sustrans are now developing the "Two tunnels" cycle route. See (2) above
8. Tucking Mill Viaduct to Wellow is now a cycle track. See (2) above
9. Midford platform might still be there but there is a pub car park beyond it.
10. Wellow - bridge removed and some of the track bed redeveloped
11. Wellow to Shoscombe - infilled cuttings and the land returned to agriculture
12. Shoscombe to Radstock is a cycle track. See (2) above
13. Radstock. Someone has rather inconsiderately built a cul de sac of houses on the track bed just east of where the station used to be
14. Radstock level crossing. See (3) above
15. Radstock to Midsomer Norton. Two bridges removed
16. Chilcompton - redevelopment of the track bed at two locations
17. Emborough - bridge missing and some redevelopment of the track bed
18. Binegar - some redevelopment of the track bed
19. Masbury. Part of the cutting is infilled, and the rest is a SSSI, Bridge removed just south of the station
20. Shepton Mallet. Complete redevelopment of the station site and for some distance southwards.
I wish I hadn't started this now

But having got this far, I may as well finish!
21. Prestleigh - somebody has nicked a viaduct
22. Evercreech - track bed redeveloped and bridges missing to the north and south
23. Evercreech junction - redeveloped and the station is a private house with infill between the platforms
24. Evercreech Junction level crossing (see 3 and 14 above)
25. Cole. Bridges and viaduct missing, much land either side returned to agriculture
26. Wincanton. Complete redevelopment of the formation right the way through the town
27. Templecombe. Thales have taken over the engine shed and all of the adjoining trackbed.
28. Stalbridge. Station site redeveloped
29. Sturminster Newton. Bridge over the Stour missing and station site redeveloped
30. Stourpaine - bridge missing
31. A350 Blandford bypass severs the line
32. Blandford - station area and south thereof redeveloped, most of a viaduct missing
33. South Blandford - Tesco has built a superstore on the track bed
34. Charlton Marshall to Spetisbury - Sustrans again ....
35. Sturminster Marshall/ Baily Gate - station area redeveloped
36. Corfe Mullen A31 crossing - see 3, 14 and 24
37. Broadstone - station area has been redeveloped
38. Broadstone to Holes Bay. Track formation now used as a road in the main and a cycle track, and the adjacent land all completely redeveloped.
It will need more than a couple of jumble sales and raffles to sort that lot out, and we haven't got to such minor issues as track, signalling, stations and motive power yet.