Locomotives, machines used to move trains, may be powered by steam, Diesel fuel, or electricity. The steam locomotive's energy is produced when wood, coal, or fuel oil burned in a "firebox" creates heat that turns the water in a boiler to steam; the steam is fed into cylinders, where the pressure it produces drives the piston s (steel rods) that move the locomotive's driving wheels. The Diesel locomotive's engine functions when air is compressed in its cylinders until its temperature is high enough to ignite the fuel that has been injected into the cylinder; the resulting power is transmitted to the driving wheels. The electric locomotive uses electricity transmitted from a power plant by special overhead wires or an electrical third rail. Around the middle of the twentieth century, Diesel locomotives began to replace their steam-driven predecessors. There are now more than 20,000 Diesel locomotives in the United States...