A smokebox is a component in the propulsion system of a steam locomotive. It is essentially the exhaust system of the locomotive, as it is responsible for collecting smoke byproducts from burned coal and releasing it through a chimney into the atmosphere. The smokebox is generally mounted at the nose of the train, just below the chimney, and it may or may not contain a blower system that helps collect the smoke and push it into and out of the box. The blastpipe is inside the box, and this component propels the exhaust outward through the chimney.
Smoke from the firebox in the locomotive passes through pipes, which heats the water boiler in the locomotive engine. This heating produces the steam that is used to propel the locomotive forward. Once that smoke passes through the pipes to heat the water, the smoke enters the smokebox for propulsion up and out of the locomotive. The blower system helps propel the smoke from the firebox, and it also helps ventilate the fire, stoking it even hotter. Once smoke reaches the smokebox, the gases and smoke vent from the locomotive, preventing overheating as well as system failure.
The blastpipe is also mounted inside the smokebox. This blastpipe collects exhaust from the steam cylinders that propel the locomotive, and pushes this exhaust through the smokebox and out of the chimney. It is most often mounted directly beneath the chimney to promote quick escape of the exhaust. The signature sound of a steam locomotive engine is due to the exhaust escaping through the blastpipe and into the smokebox.
Smokeboxes are often hinged or otherwise removable to allow a maintenance person to open the box up for cleaning and maintenance. This is the only way to access some of the components within should a problem arise. Aside from the blastpipe and other ventilation pipes, some trains feature the header of an element known as a superheater, which converts wet steam into dry steam for better propulsion and efficiency. Dry steam expands more readily than wet steam, so the propulsion properties are far greater; the superheater uses a series of headers and pipes to make the conversion, making the overall process of steam production more efficient. Often, when maintenance of the superheater is necessary, it is accessed through the smokebox. Ash buildup can affect the efficiency of all the components within the box, so it sometimes needs to be cleaned to improve function.