British Rail class D3/12 was a class of three experimental diesel-electric shunting locomotives designed by Richard Maunsell of the Southern Railway in 1937. They quickly proved their effectiveness, and although they were not immediately replicated due to the advent of the Second World War, the class were the basis for a larger class of similar locomotives introduced by Oliver Bulleid in 1949. Background and Construction Once the UK economy began to improve in the mid 1930s, the Southern Railway had further need for heavy shunting locomotives in its marshalling yards around London. The Z class steam locomotives had proved successful for this task but Maunsell wanted to compare their performance with diesel-electric alternatives. He therefore sought authorisation for the construction of three experimental locomotives for use in the busy Norwood marshalling yard where there was a continuous need for such locomotives. No private manufacturer was in a position to build the new locomotives and so they were jointly constructed by the Southern Railway's Ashford works, who constructed the framing, cabs and bodywork, and English Electric Ltd of Preston, who fitted the traction motors. The three locomotives were also provided with duplicate controls to enable them to be driven by one man from either side of the cab. They were delivered during September and October 1937. Operation After a few teething problems, the class quickly proved the advantages of diesel electric shunters in terms of effectiveness, cost-saving, and driver comfort. Although at £7,275 they cost £1,100 more to build than a Z class steam locomotive, each one saved about £1,700 per year in operating costs. They could be used continually day and night for six days each week at Norwood without any service requirement beyond refuelling. They were however found to be too slow for their secondary task of freight transfers around the congested passenger lines in the London area, although perfectly well suited to this traffic on rural lines. As a result of these experiments Maunsell's successor, Oliver Bulleid, received authorisation to build a further eight members of the class in July 1939 but this order was quickly cancelled as a result of the outbreak of the Second World War. Between 1941 and 1945 the locomotives were commandeered by the War Department for use on the Martin Mill military railway to haul cross-channel gun installations and ammunition. (Diesel locomotives were required for this task as they would not give away