Most trains running today are powered by diesel fuel, electricity from overhead wires or ground-level rails, or a combination of both. The specific power source depends on the type of train and where it operates, but the underlying goal is always the same: converting stored energy into the force that turns the wheels. Here’s how each system works.
Diesel-Electric: The Most Common Setup
Despite the name, a diesel locomotive doesn’t connect its engine directly to the wheels the way a car does. Instead, it uses a hybrid system. A large diesel engine burns fuel to spin a driveshaft at a constant speed, and that driveshaft turns a high-voltage electrical generator. The generator then sends electrical power to a traction motor at each axle, and those motors are what actually turn the wheels.
This two-step approach exists because diesel engines alone are terrible at producing the kind of low-speed, high-force pull a train needs to get moving from a dead stop. Electric motors, on the other hand, deliver massive torque right from zero. By letting the diesel engine do what it’s best at (running steadily at one speed) and handing off the hard part to electric motors, the system gets the best of both worlds. A typical freight locomotive has four of these traction motors tucked into the wheel assemblies underneath.
Diesel-electric locomotives dominate freight rail in North America. Modern freight trains can move a ton of cargo anywhere from 196 to over 1,100 miles on a single gallon of fuel, according to a Federal Railroad Administration study. That wide range depends on terrain, train length, and speed, but even the low end dramatically outperforms trucks, which top out around 167 ton-miles per gallon.
Fully Electric: Power From the Grid
Electric trains skip the onboard engine entirely. They draw power directly from the electrical grid through one of two methods: overhead wires (called catenary lines) or a third rail running alongside the track at ground level. A device on the roof of the train called a pantograph presses against the overhead wire to collect current, while third-rail systems use a contact shoe that slides along the electrified rail.
Overhead catenary systems are common on high-speed rail lines and long-distance passenger routes across Europe and Asia. Third-rail systems are more typical in subways and urban transit, where lower voltages and enclosed tunnels make overhead wires impractical. Some rail networks use both, which creates engineering challenges since the two systems may run on different types of current (direct current versus alternating current) that don’t mix well without careful electrical design.
The major advantage of fully electric trains is efficiency. There’s no fuel being burned onboard, no engine idling, and the motors convert electricity into motion with far less waste heat than a combustion engine. Electric trains can also recover energy through regenerative braking, a process where the traction motors run in reverse during deceleration, acting as generators that feed electricity back into the power grid. Research on third-rail transit systems has shown this can save up to 55% of the system’s total electrical energy consumption.
Battery-Electric: A Newer Option
Battery-powered locomotives are an emerging alternative for freight rail, particularly in regions where electrifying thousands of miles of track isn’t practical. The concept is straightforward: instead of a diesel engine and generator, the locomotive carries large battery packs that power the same type of traction motors.
Research from Lawrence Berkeley National Laboratory found that a single railcar carrying a 9-megawatt-hour battery pack could power an average freight train for about 150 miles, which happens to be the average daily distance for a Class I freight train in the U.S. Charging times have dropped significantly with newer battery technology, reaching 30 minutes to an hour for a full charge at the cell level. With a small surplus of battery cars available for swapping, operators could keep trains running around the clock without waiting for a recharge.
Magnetic Levitation: No Wheels at All
Maglev trains work on a completely different principle. Instead of wheels on rails, they float above a guideway using powerful magnetic fields, and they’re propelled forward by magnetism too. The U.S. Department of Energy describes the propulsion system as a series of electromagnetic loops built into the guideway itself, powered by alternating current. These loops create magnetic fields that simultaneously pull the train forward from the front and push it forward from behind, like passing a magnet along a line of paperclips.
Because there’s no physical contact between the train and the track, there’s no rolling friction. The only resistance is air. This is why maglev trains can reach speeds above 300 miles per hour. The tradeoff is infrastructure cost: every inch of guideway has to be built with precision-engineered electromagnets, making maglev lines vastly more expensive per mile than conventional rail.
Why Diesel Beat Steam
Steam locomotives, which burned coal or wood to boil water and drive pistons with pressurized steam, powered rail travel for over a century. They were replaced not because diesel was a newer technology, but because steam was extraordinarily wasteful. Early low-pressure steam engines converted only about 3% of the energy in their fuel into useful motion. The rest escaped as heat. Diesel-electric locomotives offered a dramatic leap in efficiency, along with the practical benefits of not needing water towers and coal depots every hundred miles. By the 1960s, steam had essentially vanished from commercial rail in the developed world.