The Somerset & Dorset Joint Railway (S & D) main line ran south from Bath Queen Square (later renamed Green Park) to Radstock, at one time the centre of the Somerset coalfield. From there, it ran over Masbury summit, at 274 m (811 ft) above sea level crossing the Mendip Hills, via Shepton Mallet and entering the catchment area of the River Stour to Wincanton and Blandford, joining the L&SWR Weymouth line at Poole. S&D trains continued to the L&SWR station at Bournemouth West.

It was jointly operated by the Midland Railway and the London and South Western Railway (L&SWR). After the 1 January 1923 Grouping, joint ownership of the S&D passed to the LMS and the Southern Railway. It was never a high speed line: its winter business was carrying freight and local passenger traffic over the Mendips, but in the summer season the S&D came into its own, carrying a heavy service of Saturday holiday trains from northern towns to Bournemouth. It always commanded a considerable loyalty from railway enthusiasts, and when it closed in 1966 it was widely mourned. Evidence of this is the number of organisarions aiming to restore all or part of it:

• The New Somerset and Dorset Railway - a land purchasing and lobbying group determined to reopen the whole S&D as a response to Peak Oil and Climate Change, blending sustainable modern and heritage transport.
• Somerset and Dorset Railway Heritage Trust - restoring Midsomer Norton station with plans to extend to Radstock and towards Shepton Mallet.
• North Dorset Railway Trust - restoring Shillingstone station with plans to extend in both directions.
• Gartell Light Railway - a two foot gauge private railway operating on part of the S&D near Templecombe, currently extending northwards.
• Somerset and Dorset Railway Trust - based at Washford on the West Somerset Railway, preserving relics and rolling stock of the S&D.
• There is also a private restoration project at Masbury station and possibilities of some S&D presence at Midford.

At Midford a few miles south of Bath the S & D came through a single line tunnel and crossed a road, a small stream and another railway line on a viaduct. This minor line was a GWR branch from Limpley Stoke to Hallatrow and was itself carried on a smaller viaduct over the road. The S & D viaduct was 154 m (505 ft) long built of brick and stone. This unusual location was featured in one of the opening sequences of the film ‘˜The Titfield Thunderbolt'.