A steam locomotive is a locomotive which is powered by steam. From the early 1800s through the beginning of the 20th century, steam locomotives dominated railways around the world. Even after internal combustion engines were developed for cars, external combustion power in the form of steam locomotives continued to be the energy source of preference on railways. Eventually, electric and diesel locomotives which could perform comparably were developed, and today steam locomotives are rarely seen in use.
In a steam engine, fuel such as oil, wood, or coal is used to heat a boiler filled with water. The heat causes high pressure steam to develop, and the steam pushes pistons. In the case of a steam locomotive, the pistons push the wheels of the locomotive, allowing it to serve as a source of motive power for the train. For a standard train, a single steam locomotive was often sufficient to meet the train's power needs. However, trains which had to travel up steep grades or handle heavy loads sometimes employed two or more locomotives.
There are some obvious problems associated with using a steam locomotive. Boiler explosions were historically a problem on many railroads. The boiler could explode as a result of a dangerous pressure buildup, poor maintenance, or poor construction, and such explosions could be deadly to people in close proximity. Running a steam locomotive also requires carrying substantial amounts of fuel, and the train must periodically take on water to refill the boiler, since much of the fluid is lost when waste steam is vented.
The earliest steam engines were developed in the late 1700s. By the early 1800s, the obvious applications for railroading could be seen, and companies started producing steam locomotives. The introduction of the steam locomotive was revolutionary for the transport of humans and goods. People were no longer required to travel at the speed of draft animals such as horses and trains were able to carry very heavy loads which would have been beyond the capacity of working animals.
Many people are familiar with the chuffing noise associated with steam locomotives, especially when they start to move. In fact, one of the nicknames for the steam locomotive, “choo choo,” is imitative in origin and references the distinctive sound made by a locomotive getting underway. This noise is caused by the slow movement of the pistons as the engine warms up. Once the train gets going, the piston moves more quickly and the noise is less noticeable. People may note that the chuffing noise increases as trains slow to move around curves or enter stations.