The British Railways Class D3/7 is a class of 0-6-0 diesel electric shunting locomotives built as LMS Nos. 7080–7119. The class were built from May 1939 through to July 1942 by the London, Midland and Scottish Railway at their Derby Works using a diesel electric transmission supplied by English Electric.
They are a modified version of the 1934-vintage Class D3/6 (LMS 7069-7079) diesel shunters based on the English Electric 6K diesel engine of , but have jackshaft transmission necessitating a significant increase in body length. The D3/6 had two axle-hung traction motors instead, and this feature became commonplace in more modern designs built after World War II.
Design
The locomotives are built to the specifications of LMS CME W.A. Stanier for general and hump shunting at the company's Derby Works using engine and electric equipment supplied by the English Electric Company.
Powertrain
The engine and generator are supported on girders attached to the main frame via a three-point suspension with rubber vibration absorbing pads; the generator is to the rear of the engine. The locomotive body is compartmentalised, with side doors and a sliding roof allowing access.
The main generator is a direct current machine. Main control was via notched engine speed control (350, 465, 590, and 680 rpm), with finer control via secondary lever. The engine is fitted with a governor preventing overspeed, and electric overload protection.
The locomotive's mechanical transmission consists of a single, frame-mounted traction motor powering a jackshaft drive via a reduction gear; the jackshaft drives all three driving axles via connecting rod and coupling rods.
Auxiliaries
An 80-volt battery is used to energise the main generator, as well as powering the electric control system and lights, and is also used to start the engine with the generator in motor mode. The battery is automatically charged when the engine is idling. An air compressor is powered via a belt drive from the generator. Compressed air operates the Westinghouse double-wheel tread brakes, as well as the sanding equipment and the whistle.