The 4073 or Castle Class are 4-6-0 steam locomotives of the Great Western Railway, built between 1923 and 1950. They were designed by the railway's Chief Mechanical Engineer, Charles Collett, for working the company's express passenger trains. They could reach speeds of up to 100 mph (160 km/h).

A 4-6-0 steam locomotive, under the Whyte notation for the classification of steam locomotives by wheel arrangement, has four leading wheels on two axles in a leading bogie and six powered and coupled driving wheels on three axles with the absence of trailing wheels.