The steeply graded Somerset and Dorset route into the Somerset coal field demanded powerful and robust locomotives. designed at Derby specifically for the route these 7Fs were large and modern engines in 1914 and the fleet was expanded in 1925 to 11
The Somerset and Dorset Joint Railway (S&DJR) Class 7F 2-8-0 locomotive was designed for heavy freight. Eleven were built in two batches in 1914 and 1925. James Clayton, the draughtsman at Derby was given a free hand to design the engine, and produced something unlike any other Derby-designed locomotive of the time. At its time of introduction, the Class 7F was one of the most powerful mineral engines in the country.
The design used the G9AS boiler from the Midland Compounds, with a Belpaire firebox and Walschaerts valve gear. A leading pony truck was added, to distribute the weight, making it a 2-8-0 arrangement. The cylinders were mounted high on the frame, and sloped, to avoid fouling platform faces. The S&DJR was highly graded through the Mendip Hills and required power over and above what was available from the Midland's small engines. Because of this, Clayton provided two steam brake cylinders on the engine and a further one on the tender. The Midland-style tenders differed from usual practice in that they were not equipped with water pick-up apparatus as the S & D system was devoid of troughs.
Two locomotives have survived into preservation, 88 (53808) still in Somerset, albiet onthe former GWR West Somerset Railwayand 89 (53809) at the Midland Railway Centre, near Derby, home of the Midland Railway.