Class D3/6 were diesel shunters built in 1935 by English Electric for the London, Midland and Scottish Railway. They were the first of a new design of diesel shunters based on the English Electric 6K of 350 horsepower (260 kW) diesel engine. Similar prototypes were built for the Southern Railway (SR1-3), Great Western Railway (GWR2), and London and North Eastern Railway (8000-8004). They were all substantially similar, with the long bonnet ("nose"), 0-6-0 wheel arrangement, EE 6K prime mover and 2 axle-hung traction motors. This configuration was repeated in most subsequent large diesel shunters built in Britain, such as the British Rail Class 11 and even more numerous British Rail Class 08 that entered service after World War II.
One demonstrator and ten production locomotives were built, numbered 7079 and 7069-7078 by the LMS. Three of these later became British Rail numbers 12000–12002 (previously 7074/76/79 respectively). The remainder were sold to the British War Department in 1940 and were shipped to France during World War II. Of these the first production locomotive, no. 7069, survived the war and was used in industrial service in France. It was later repatriated for preservation and is currently located at the Berkeley Vale Railway.