despite the success of the superheating the d boiler, it was still somewhat limited in steam raising capabilities. in 1922, a new design of 2-8-0 branch line goods locomotive, the k class was introduced, with noticeably superior boiler performance to that of the d. in 1929, a d class locomotive was rebuilt with a larger boiler derived from the k class design. based on the success of the rebuild, a further 93 dor d class locomotives were converted between 1929 and 1947.