Steam locomotives use a basic concept of propulsion, built upon with other features. The basic structure comprises of a firebox, boiler, and cylinders. Fire is built up in the firebox, where the heat is transferred through the boiler via many pipes. The heat heats up the water which becomes steam. The steam goes through another pipe to a dome which regulates steam flow, which controls the steam. It goes to the cylinders which pushes on pistons, which are connected to driver wheels, which moves the locomotive and what it's pulling. One of the most often used additions are pilot wheels, unpowered bogies which help guide the powered drivers. Another are rear bogies which support the cab and firebox if the need to arise. Yet another are tenders which are permanently coupled cars which carry water and coal, or anything else that could produce heat. More variations include mallets (2 sets of drivers), Garrets (Same concept, 2 boilers) and more.