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.
Steam locomotive
More-powerful locomotives tend to be longer, but long rigid-framed designs are impractical for the tight curves frequently found on narrow-gauge railways. Various designs for articulated locomotives were developed to overcome this problem. The Mallet and the Garratt were the two most popular, both using a single boiler and two engines (sets of cylinders and driving wheels). The Garratt has two power bogies, whereas the Mallet has one. There were also a few examples of triplex locomotives that had a third engine under the tender. Both the front and tender engines were low-pressure compounded, though they could be operated simple (high-pressure) for starting off. Other less common variations included the Fairlie locomotive, which had two boilers back-to-back on a common frame, with two separate power bogies.