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A locomotive boiler is the central part of the engine for any steam-powered locomotive train. Boilers similar to the locomotive boiler, which is also called a fire-tube boiler, can power other vessels, including ships. The fire-tube boiler, however, is most commonly identified with steam locomotives, used around the world since the early 19th century. Steam locomotives are still in use in the 21st century, often for historical or novelty travel, despite the emergence of more modern diesel and electric locomotive engines.
Steam engines were first developed in the late 1700s and early 1800s. The principle of steam power is that steam from heated water is a form of pressurized gas. Safely and properly directed, this gas can generate massive levels of energy. A steam locomotive applies this energy to move a train long distances at a high rate of speed. The locomotive boiler is the most important part of the process, creating the steam and directing it to power the locomotive’s wheels.
This type of boiler consists of a series of connected tanks and tubes designed to operate at extremely high temperatures. An accessible tank called a firebox contains the burning fuel, usually coal. Although any combustible material could be used, coal is the most efficient fuel for this kind of engine. Exhausted fuel falls through a grate in the bottom of the firebox as ash for later disposal. The heat from the firebox travels through a series of large and small tubes to the front of the boiler. These tubes are called flues or fire tubes, giving the fire-tube boiler its name.
The fire tubes pass through a central tank filled with water, and heat from the tubes converts the water to steam. In some versions of the locomotive boiler, this steam is heated yet again, making it superheated steam, which is safer and more efficient than ordinary steam. In nature, this steam would escape quickly and violently, but the locomotive boiler is designed to contain it safely and direct it toward the locomotive engine’s cylinders and pistons. These, in turn, move the locomotive’s coupling rods, which are connected to the driving wheels and put the train in motion.
This complicated and potentially dangerous process was perfected when steam locomotives were the primary means of land transportation. Properly maintained, a locomotive boiler will last for decades, and many steam locomotives from the early 20th century are still in use. The steam locomotive has played a key role in dozens of films, from Buster Keaton’s 1927 silent classic The General to the 1990s time-travel comedy Back to the Future Part III.