Diesel locomotives have seen limited use on the London Underground, largely because exhaust gases cannot be discharged when the vehicles are working in tunnels. A prototype diesel engine numbered DEL120 was built in 1939 from two 1915 stock motor cars, which was expected to be part of a batch of ten, but experience with battery locomotives showed that these were a better alternative. Three 0-6-0 diesels (DL81-DL83) were obtained in 1971, to replace the last steam engines, but were too short to operate the signalling system, and too heavy for some of the bridges. In 1996, fourteen diesels (numbered 1 through 14, but also given female names) were supplied by Schöma of Germany, which were used during the construction of the Jubilee line tunnels. They were fitted with exhaust scrubbers, to enable them to work in the tunnels. To speed up track renewals on the subsurface lines, Class 66 locomotives have been hired in since 2006 to handle permanent way trains, but again suffer from being too heavy for some of the bridges. Because they are not fitted with tripcock safety devices, and pull trains much longer than the signalling system is designed for, they are restricted to slow speed running. DEL120 Design The design of a prototype diesel locomotive which could work through London's tube tunnels was first considered in 1936, and work began on its construction at Acton Works in 1939. It was designed to use its diesel engine when in the open air, but to draw current from the third and fourth rail when underground. Two Brush built 1915 Stock (also known as 'Ealing Stock') driving motors, numbered 3937 and 3941, which had been withdrawn from the Central London line, were converted into the electro-diesel locomotive. The work involved scrapping the passenger saloons and joining the two driving ends together to form a double ended locomotive. The central bay held a six-cylinder two-stroke diesel engine, manufactured by Petters, which was coupled to a direct current generator. Construction The prototype, which was the first electro-diesel locomotive to be constructed in Britain, was completed in November 1940, and was painted in Metropolitan 'Lake', with gold lining. It was intended to build ten similar locomotives, to replace the fleet of steam engines, and a further 18 withdrawn motor cars were stored at Cockfosters depot with this in mind. The construction of further locomotives was delayed by the outbreak of the