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.
British Rail Class 73
Several locomotives have been preserved on heritage railways, where perforce they run on their diesel engines: no preserved line has third rail electrification. They are particularly popular because they have a small diesel engine and hence are efficient with speeds normally limited to 25 mph, yet they are big mainline locomotives and thus more attractive than diesel shunters with similar-sized engines. Of note are the first built locomotive, No. 73001 (an ex-Merseyrail Class 73/9), and the former Pullman locomotive, No. 73101.