Diesel locomotive
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train — something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.
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.