London Underground battery-electric locomotives

London Underground battery-electric locomotives, most commonly known as battery locomotives are used for hauling engineers' trains, as they can operate when the electric traction current is switched off. The first two locomotives were built in 1905 for the construction of the Great Northern, Piccadilly and Brompton Railway, and their success prompted the District Railway to buy two more in 1909, which were the only ones built to the loading gauge of the subsurface lines.

The locomotives have a cab at each end and are built to the standard 'Tube' loading gauge so that they can work over all lines on the London Underground network. They are equipped with buffers and draw hooks, for coupling to standard main line vehicles

The body sides panel are  louvers to allow ventilation around the batteries, the panels are hinged to allow the batteries to be removed. Following the withdrawal of steam engines, the vehicles often worked on open sections of line,

The traditional use of these locomotives has been to haul trains using power from the rails until they reach the area where work is to be undertaken, where they switch to battery operation if the traction supply has been isolated. They are also used for transporting diesel powered equipment such as track tamping machines through tunnel sections. Normally one locomotive is marshaled at each end of the train, allowing the train to be reversed easily.