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.
Electric locomotive
In North America, the flexibility of diesel locomotives and the relative low cost of their infrastructure has led them to prevail except where legal or operational constraints dictate the use of electricity. An example of the latter is the use of electric locomotives by Amtrak and commuter railroads in the Northeast. New Jersey Transit New York corridor uses ALP-46 electric locomotives, due to the prohibition on diesel operation in Penn Station and the Hudson and East River Tunnels leading to it. Some other trains to Penn Station use dual-mode locomotives that can also operate off third-rail power in the tunnels and the station. Electric locomotives are planned for the California High Speed Rail system.