Diesel fuel has become the preferred fuel for railroad locomotive use due to its lower volatility, lower cost, and common availability. The diesel engine (A) is the main component of the diesel-electric locomotive. It is an internal combustion engine comprised of several cylinders connected to a common crankshaft.

Most modern non-electric trains run on diesel-electric systems because diesel engines are incredibly efficient at moving massive loads over long distances without the need for expensive infrastructure like overhead wires. In this setup, a large diesel engine doesn't actually turn the wheels; instead, it acts as a giant mobile generator that produces electricity. This electricity then powers "traction motors" at the axles, which provide the immense torque needed to start a heavy train from a standstill. Diesel is preferred over gasoline because it is more energy-dense and less volatile, making it safer for industrial use. While many countries in Europe and Asia have "electrified" their main lines to pull power directly from the grid, diesel remain the "workhorse" for freight and rural passenger routes in North America and Africa because it is far more cost-effective than installing thousands of miles of electrical cabling through remote terrain.