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.
Henry Roe Campbell
One of the most visually striking features of a steam locomotive is its "wheel arrangement" which is largely a function of its intended application and purpose. The fundamental principle of design for 19th century steam locomotives was tractive force which relies on adhesion. A switching locomotive for example, is relatively low-powered but with a high starting tractive effort for getting heavy cars rolling quickly. These locomotives are geared to produce high torque but are restricted to low top speeds and have small diameter driving wheels. Switchers are rail analogs to tugboats. For higher speeds, larger boilers were required which in turn led to the development of lead trucks.
Prior to 1832, the typical locomotive for general road service on American roads was a 2-2-0 which could be built with either inside or outside steam cylinders. The critical element of the design was the single pair of driving wheels placed either in front of the firebox, as in the Norris engines, or in back of the firebox, as patented in 1834 by E. L. Miller and used extensively by in the Baldwin engines.