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.

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.

It was intended to build ten similar locomotives, to replace the fleet of steam engines, and a further 18 withdrawn motor cars were stored at Cockfosters depot with this in mind.

They were painted dark green and operated at Neasden and Lillie Bridge depots where they replaced the last of the ex-GWR pannier tank steam locomotives.

Since this made their use impracticable, each was permanently coupled to a tender, which consisted of a bogie removed from a redundant District line Q Stock car.

The weight of the bogie was increased to 17 tons, and it was fitted with tripcocks and sleet brushes, so that it could be used for clearing snow and ice from the current rails.

Movement outside of the depots was severely restricted when the civil engineering department decided that the short wheelbase and 16-ton axle loading might cause overloading problems on a number of bridges.

In connection with the construction of the Jubilee Line Extension project, fourteen diesel-hydraulic locomotives were purchased from Schöma of Germany to assist in equipping the tunnels prior to electrification.

Weighing 33.88 tonnes each, they are 28 feet (8.5 m) long, and are powered by 500-horsepower (370 kW) six-cylinder inline diesel engines, which give them a maximum speed of 31 mph (50 km/h).

[citation needed] Metronet contracted five newly built Class 66 locomotives from GB Railfreight in order to speed up track replacement works on the London Underground network.