Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
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.