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.
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.