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.
Tender (rail)
The largest steam locomotives are semi-permanently coupled by a drawbar to a tender that carries the water and fuel. The fuel source used depends on what is economically available locally. In the UK and parts of Europe, a plentiful supply of coal made this the obvious choice from the earliest days of the steam engine. Until around 1850 in the United States, the vast majority of locomotives burned wood until most of the eastern forests were cleared. Subsequently, coal burning became more widespread, and wood burners were restricted to rural and logging districts.