Dieselisation
Dieselisation took place largely because of the tremendous reduction in operating costs it allowed. Steam locomotives require large pools of labour to clean, load, maintain and run. They also require extensive service, coaling and watering facilities. This was their biggest inferior measure as compared to the diesel locomotive in the number of ton-miles or passenger traffic miles either could run. Diesels could and did have a significantly higher initial price per unit-horsepower delivered. However, their far greater range between fueling stops, the absence of water stops, and the much higher unit availability between inspection repair and maintenance stops, were far better than steam. Diesels simply required significantly less time and labour to operate and maintain. Diesel power was also more scalable to power requirements, owing to the control systems that allowed multiple units to be controlled by one operator. "Double header" steam power required a crew for each locomotive. Initially, diesel locomotives were less powerful than the typical steam locomotives. Between the late 1930s and the late 1950s the power available with diesel locomotive engines roughly doubled, although the most powerful steam locomotives ever built still exceeded the power of the most powerful diesel locomotives from the late Twentieth Century.
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.