Diesel locomotive
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train — something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.
London Underground diesel locomotives
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.