This is a route where where a few modest infrastructure improvements could make a major improvement for passengers.
But the real Achilles heel is its poor siting of stations and routing.
Bristol to Frome is a decidedly long way round, and the long-gone line through Radstock was about 15 miles shorter. If the junction at Frome had faced the other way it might have formed an effective alternative. However it was steeply graded, the advantage of the line round by Trowbridge was being pretty level and suiting the days when there was much heavy freight from South Wales to southern England sent this way.
But the siting of the stations ...
Trowbridge is actually on the main street. But the only one like this on the line.
Westbury is well out of town, to such an extent that when the avoiding line was built in the 1930s, significantly passing the junction station, it actually passed between the station and the town.
Frome station is by the last houses on the Warminster road before it heads out into the country.
Bruton is likewise on the periphery.
Castle Cary station is in the middle of nowhere. The line actually performs an arc around the town at a radius of over a mile. The so-called Station Road, the B3152, has little or no pavement or street lighting through the fields, so you could never send schoolchildren along it.
Yeovil has the ridiculous situation of being served by two stations on two unconnected routes, both of which are beyond walking distance from the town. It used to have a third, Yeovil Town, centrally situated, also the start of the old Taunton line, when there were shuttle trains to both. Incidentally, you can actually see the Weymouth trains passing, at speed, from the east end of the Junction platforms. But just try getting to them.
Thornford has to be the remotest of all, just surrounded by fields.
Yetminster is, for once, well-sited.
Chetnole is a repeat of Thornford.
Maiden Newton is OK.
Dorchester West is actually sited better than Dorchester South relative to the centre of town, but having such an irregular service to Weymouth in comparison, nobody would go there for heading south.
chords and new lines that even the Victorians at the height of the railway mania couldn't actually justify building
I think the only serious one is the two chords into Yeovil Junction. Which the Victorians did build, as far as the earthworks go, which can still be readily seen on Google Earth. It was just that GWR-LSWR rivalry prevented them being used.