The Duke class consisted of 60 4-4-0 express locomotives designed by William Dean for service in Devon and Cornwall, mainly between Newton Abbot and Penzance. After four Armstrong class prototypes built in 1894, the Duke class was introduced in 1895. 60 had been built at Swindon by 1899. They could be recognized by curved outside frames and had a parallel domeless boiler. Most had a round-topped firebox, in contrast to the last four with a raised Belpaire firebox. Their slide valves were mounted underneath the cylinders to reduce wear. The first 25 had short tenders so that they could fit onto the existing turntables.

Between 1906 and 1910, 19 were rebuilt with a domeless tapered boiler and became part of the Bulldog class. Until 1923, all others received a boiler with a flush-topped firebox and a pressure of 180 instead of 160 psi. No. 3258 had been equipped with a superheater in 1896. Between 1911 and 1946, all others but two also received a superheater. In 1929, the boiler of a Duke class locomotive was mounted onto the frames of a Bulldog class locomotive. This reduced the axle load so that it could be used on the Cambrian section. Between 1936 and 1939, 29 more were used for the same rebuild which were called Earl class. Only ten unrebuilt Duke class locomotives could be taken over by British Railways. These were withdrawn between 1949 and 1951.