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.
District Railway steam locomotives
When in 1871 the District Railway needed its own locomotives, they ordered 24 condensing steam locomotives from Beyer Peacock similar to the A Class locomotives the Metropolitan Railway was using on the route. The 4-4-0 tank locomotives had x cylinders, diameter driving wheels and weighed 42 ton 3 cwt in working order. The boiler pressure was , the front wheels were on a Bissel truck and fitted with bunker. As they were intended for an underground railway, the locomotives did not have cabs. To reduce smoke underground, at first the Metropolitan had used coke, but after 1869 this was changed to smokeless Welsh coal. The only obvious differences were a different chimney style and a bent-back top to the weatherboard, and the back plate of the bunker was raised to provide protection when running bunker first.