Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
Dieselisation
In rail transport, dieselisation refers to the replacement of the steam locomotive or electric locomotive with the diesel-electric locomotive (often referred to as a "diesel locomotive"), a process which began in the 1930s and is now substantially complete around the world.
The replacement of either steam or diesel haulage with electric locomotives is known as electrification. Whereas the benefit of replacing steam traction is indisputable, there is some dispute as to whether it is best replaced by dieselisation or electrification. Electrification has a high initial capital cost but the operating costs are lower. The overall savings depend on the effect of the investment cost compared with the savings due to lower operational and maintenance costs and the influence of better acceleration and tractive effort on railroad throughput. These are obviously different for e.g. urban networks and very long-distance networks with low frequencies. However, many railway commentators are increasingly suggesting that the ability of railways to operate with electricity not produced from fossil fuels may offer a decisive advantage over diesel power.