How Diesel Locomotives Work

By: Karim Nice & Talon Homer  | 
Norfolk Southern freight train
A Norfolk Southern freight train in Pennsylvania pulled by a GE diesel locomotive, travels east at the railroad tracks Jan. 27, 2021. Ben Hasty/MediaNews Group/Reading Eagle via Getty Images

Diesel engines are used in many types of vehicles, including locomotives. Diesel engines have a fuel efficiency 20 percent greater thermally than a gas engine. This means a 20 percent increase in fuel economy and therefore lower operating costs than those of a gas engine. Diesel engines also last longer than gas engines because they run at a much slower rpm (revolutions per minute) rate than gas engines do.

The hybrid diesel locomotive is an incredible display of power and ingenuity. It combines some great mechanical technology, including a huge, 12- to 16-cylinder, two-stroke diesel engine, with some heavy-duty electric motors and generators, throwing in a little bit of computer technology for good measure.


The locomotives weigh between 100 and 200 tons (91,000 and 181,000 kilograms) and are designed to tow passenger-train cars at speeds of up to 125 miles per hour (200 kph). Siemens' modern engines produce up to 4,200 horsepower, and the generator can turn this into almost 4,700 amps of electrical current. The drive motors use this electricity to generate around 60,000 lb-ft of torque. There is also a secondary diesel engine and generator to provide electrical power for the rest of the train. This generator is called the head-end power unit, producing between 500 and 700 kilowatts (kW) of electrical power.

This combination of diesel engine and electric generators and motors makes the locomotive a hybrid vehicle. In this article, we'll start by learning why locomotives are built this way and why they have steel wheels. Then we'll look at the layout and key components.


Why Hybrid? Why Diesel?

The 3,200-horsepower engine drives the main generator.

The main reason why diesel locomotives are hybrid is because this combination eliminates the need for a mechanical transmission, as found in cars. Let's start by understanding why cars have transmissions.

Your car needs a transmission because of the physics of the gasoline engine. First, any engine has a redline — a maximum rpm value above which the engine cannot go without exploding. Second, if you have read How Horsepower Works, then you know that engines have a narrow rpm range where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower between 5,200 and 5,500 rpms. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so that the engine can stay below the redline and near the rpm band of its best performance (maximum power).


The five-to-10-speed transmission on most cars allows them to go 110 mph (177 kph) or faster with an engine-speed range of 500 to 6,000 or higher rpm. Diesel engines have a much slower operating speed than gasoline, and that goes double for the massive ones used in locomotives. The large displacement diesel engine tops out at about 2,100 rpm, or lower. With a speed range like this, a locomotive would need 20 or 30 gears to make it up to 110 mph.

A gearbox like this would be huge (it would have to handle 4,200 horsepower), complicated and inefficient, and create a possible point of mechanical failure. It would also have to provide power to four sets of wheels, which would add to the complexity.

By going with a hybrid setup, the main diesel engine can run at a constant speed, turning an electrical generator via driveshaft. The generator sends electrical power to a traction motor at each axle, which powers the wheels. The traction motors can produce adequate torque at any speed, from a full stop to 125 mph (200 kph), without needing to change gears.

Why Diesel?

Diesel engines are more efficient than gasoline engines, and when moving literal tons of freight or passengers, efficiency is paramount. Train manufacturer CSX estimates that their fleet moves 1 ton (0.9 metric tons) of cargo an average of 492 miles (791 kilometers) per 1 gallon (4 liters) of fuel, making locomotives four times as efficient as moving goods on roadways. Diesel-electric systems are also five times more efficient than the old steam engine locomotives, which is why diesel entirely replaced steam in the early 20th century.

Diesel also has seen some competition from fully electric trains, which pull directly from a power grid as they drive. This method is several times more efficient than burning any kind of onboard fuel to produce energy. Electric locomotives are especially popular in Europe and Asia, but the changeover in the U.S. has been slow. Probable causes are that electric trains require their own specialized infrastructure to operate, and old locomotives can be in service for multiple decades before retirement. For the time being, diesel remains the standard. A few passenger railways have however been electrified in the States, including Amtrak's northeast corridor and California commuter rail.


Steel Wheels

Amtrak long-distance train
An Amtrak long-distance train travels through the lush forests and wildflower meadows of the Pacific Northwest. Amtrak

Ever wonder why trains have steel wheels, rather than tires like a car? It's to reduce rolling friction. When your car is driving on the freeway, about 4-7 percent of its potential energy is lost to the rolling resistance of the tires. Tires bend and deform a lot as they roll, which uses a lot of energy.

The amount of energy used by the tires is proportional to the weight that is on them. Since a car is relatively light, this amount of energy is acceptable (you can buy low rolling-resistance tires for your car if you want to save a little gas).


Since a train weighs thousands of times more than a car, the rolling resistance is a huge factor in determining how much force it takes to pull the train. The steel wheels on the train ride on a tiny contact patch — the contact area between each wheel and the track is about the size of a dime.

By using steel wheels on a steel track, the amount of deformation is minimized, which reduces the rolling resistance. In fact, a train is about the most efficient way to move heavy goods.

The downside of using steel wheels is that they don't have much traction. In the next section, we'll discuss the interesting solution to this problem.



Traction when going around turns is not an issue because train wheels have flanges (projecting rims around the wheels) that keep them on the track. But traction when braking and accelerating is an issue.

A locomotive can generate more than 60,000 lb-ft of torque. But in order for it to use this torque effectively, the eight wheels on the locomotive have to be able to apply it to the track without slipping. The locomotive uses a neat trick to increase the traction.


In front of each wheel is a nozzle that uses compressed air to spray sand, which is stored in two tanks on the locomotive. The sand dramatically increases the traction of the drive wheels. The train has an electronic traction-control system that automatically starts the sand sprayers when the wheels slip or when the engineer makes an emergency stop. The system can also reduce the power of any traction motor whose wheels are slipping.

Now let's check out the layout of the locomotive.


The Layout: Main Engine and Generator

Diagram showing the layout of the typical diesel locomotive.
Diagram showing the layout of the typical diesel locomotive. Outline2Design/HowStuffWorks

Nearly every inch of the 54-foot (16.2-meter) locomotive is tightly packed with equipment.

Main Engine and Generator

The giant two-stroke, turbocharged engine and electrical generator provide the huge amount of power needed to pull heavy loads at high speeds. Cummins' locomotive engine weighs over 24,000 pounds (10,886 kilograms). The generator and electric motors add more mass on top of that. We'll talk more about the engine and generator later.



The cab of the locomotive rides on its own suspension system, which helps isolate the engineer from bumps. The seats have a suspension system as well. Inside the cab is a small working space with only a few seats. Usually the cab is only occupied by an engineer and a conductor.


Also known as bogies, the trucks are the complete assembly of two axles with wheels, traction motors, gearing, suspension and brakes. We'll discuss these components later.

Head-end Power Unit

The head-end power unit (HEP) consists of another big diesel engine, which itself can make 3,000-4,000 horsepower. It tends to spin even slower than the main engine, maxing out at about 1,000 rpm. The engine drives a generator that provides 480-volt, 3-phase AC power for the rest of the train. Many HEPs provide over 500 kilowatts of electrical power to the rest of the train, to be used by the electric air conditioners, lights and kitchen facilities.

By using a completely separate engine and generator for these systems, the train can keep the passengers comfortable even if the main engine fails. It also decreases the load on the main engine. Additionally, many modern locomotives have electronic systems that allow power from the secondary engine to be sent to the traction motors, or power from the main engine to the HEP, depending on current energy needs.

Fuel Tank

This huge tank in the underbelly of the locomotive holds up to 5,500 gallons (20,820 liters) of diesel fuel, plus an additional 300 gallons (1,135 liters) of coolant, and 250 gallons (946 liters) of engine oil. The fuel tank is compartmentalized, so if any compartment is damaged or starts to leak, pumps can remove the fuel from that compartment.


The locomotive operates on a nominal 64-volt electrical system. The locomotive has eight 8-volt batteries, each weighing over 300 pounds (136 kilograms). These batteries provide the power needed to start the engine (it has a huge starter motor), as well as to run the electronics in the locomotive. Once the main engine is running, an alternator supplies power to the electronics and the batteries.

Let's take a more detailed look at some of the main systems on the locomotive.


The Engine and Generator

Inside a diesel locomotive engine.

The main engine in this locomotive is a Caterpillar EMD 710 series engine. The "710" means that each cylinder in this turbocharged, two-stroke, diesel V-12 has a displacement of 710 cubic inches (11.6 liters). That's more than double the size of most of the biggest gasoline V-8 car engines — and we're only talking about one of the 12 cylinders in this 3,300-hp engine.

So why two-stroke? Even though this engine is huge, if it operated on the four-stroke diesel cycle, like most smaller diesel engines do, it would only make about half the power. This is because with the two-stroke cycle, there are twice as many combustion events (which produce the power) per revolution. It turns out that the diesel two-stroke engine is really much more elegant and efficient than the two-stroke gasoline engine. See How Diesel Two-Stroke Engines Work for more details.


You might be thinking, if this engine is about 24 times the size of a big V-8 car engine, and uses a two-stroke instead of a four-stroke cycle, why does it only make about 10 times the power? The reason is that this engine is designed to produce 3,300 hp continuously, and it lasts for decades. If you continuously ran the engine in your car at full power, you'd be lucky if it lasted a week.

Here are some of the specifications of this engine:

  • Number of cylinders: 12
  • Compression ratio: 16:1
  • Displacement per cylinder: 11.6 liters (710 in3)
  • Cylinder bore: 230 millimeters (9.2 inches)
  • Cylinder stroke: 279 millimeters (11.1 inches)
  • Full speed: 900 rpm
  • Normal idle speed: 200 rpm

The engine spins all this torque through the driveshaft into the high-voltage generator. The electricity produced is then sent to the four massive electric motors, located in the trucks.


The Trucks: Propulsion & Suspension

The trucks have several jobs.

The trucks are the heaviest things on the train — each one can weigh over 20,000 pounds (9,700 kilograms). The trucks do several jobs. They support the weight of the locomotive. They provide the propulsion, the suspensions and the braking. As you can imagine, they are tremendous structures.


The traction motors provide propulsion power to the wheels. There is one on each axle. Each motor drives a small gear, which meshes with a larger gear on the axle shaft. This provides the gear reduction that allows the motor to drive the train at speeds of up to 125 mph.


Each motor can weigh over 6,600 pounds (3,100 kilograms) and draw more than 700 amps of electrical current.


The trucks also provide the suspension for the locomotive. The weight of the locomotive rests on a big, round bearing, which allows the trucks to pivot so the train can make a turn. Below the pivot is a huge leaf spring, or set of coil springs, that rest on a platform. The platform is suspended by four, giant metal links, which connect to the truck assembly. These links allow the locomotive to swing from side to side.

The weight of the locomotive rests on the springs, which compress when it passes over a bump. This isolates the body of the locomotive from the bump. The links allow the trucks to move from side to side with fluctuations in the track. Some trains also incorporate pneumatic suspensions that smooth out the ride and aid in passenger comfort. The track is not perfectly straight, and at high speeds, the small variations in the track would make for a rough ride if the trucks could not swing laterally. The system also keeps the amount of weight on each rail relatively equal, reducing wear on the tracks and wheels.


The Trucks: Braking

The brakes are similar to drum brakes on a car.

Braking is provided by a mechanism that is similar to a car drum brake. An air-powered piston pushes a pad against the outer surface of the train wheel.

In conjunction with the mechanical brakes, the locomotive has dynamic braking. In this mode, each of the four traction motors acts like a generator, using the wheels of the train to apply torque to the motors and generate electrical current. The torque that the wheels apply to turn the motors slows the train down (instead of the motors turning the wheels, the wheels turn the motors). The current generated is routed into a giant resistive mesh that turns that current into heat. A cooling fan sucks air through the mesh and blows it out the top of the locomotive — effectively the world's most powerful hair dryer. Using this method of deceleration saves a lot of wear from the mechanical brakes over time.


Fully electric trains, as well as smaller vehicles like electric and hybrid cars, use a system called regenerative braking. Instead of excess energy being dissipated as heat, it's sent back into the powerline or battery, improving efficiency.

On the rear truck there is also a hand brake — yes, even trains need hand brakes. Since the brakes are air powered, they can only function while the compressor is running. If the train has been shut down for a while, there will be no air pressure to keep the brakes engaged. Without a hand brake and the failsafe of an air pressure reservoir, even a slight slope would be enough to get the train rolling because of its immense weight and the very low rolling friction between the wheels and the track.

The hand brake is a crank that pulls a chain. It takes many turns of the crank to tighten the chain. The chain pulls the piston out to apply the brakes.


Driving a Locomotive

The cab stairs of a locomotive

You don't just hop in the cab, turn the key and drive away in a diesel locomotive. Starting a train is a little more complicated than starting your car.

The engineer climbs an 8-foot (2.4-meter) ladder and enters a corridor behind the cab. They engage a knife switch (like the ones in old Frankenstein movies) that connects the batteries to the starter circuit. Then the engineer flips about a hundred switches on a circuit-breaker panel, providing power to everything from the lights to the fuel pump.


The locomotive circuit breaker

Next, the engineer walks down a corridor into the engine room. They turn and hold a switch there, which primes the fuel system, making sure that all the air is out of the system. They then turn the switch the other way and the starter motor engages. The engine cranks over and starts running.

Next, they go up to the cab to monitor the gauges and set the brakes once the compressor has pressurized the brake system. They can then head to the back of the train to release the hand brake.

Finally, they can head back up to the cab and take over control from there. Once they have permission from the conductor of the train to move, they engage the bell, which rings continuously, and sounds the airhorns twice (indicating forward motion).

The air horn

The throttle control has eight positions, plus an idle position. Each of the throttle positions is called a notch. Notch 1 is the slowest speed, and notch 8 is the highest speed. To get the train moving, the engineer releases the brakes and puts the throttle into notch 1.

Putting the throttle into notch 1 engages a set of contactors (giant electrical relays). These contactors hook the main generator to the traction motors. Each notch engages a different combination of contactors, producing a different voltage. Some combinations of contactors put certain parts of the generator winding into a series configuration that results in a higher voltage. Others put certain parts in parallel, resulting in a lower voltage. The traction motors produce more power at higher voltages.

As the contactors engage, the computerized engine controls adjust the fuel injectors to start producing more engine power.

The brake and throttle controls

The brake control varies the air pressure in the brake cylinders to apply pressure to the brake shoes. At the same time, it blends in the dynamic braking, using the motors to slow the train down as well.

A computerized readout displays data from sensors all over the locomotive. It can provide the engineer or mechanics with information that can help diagnose problems. For instance, if the pressure in the fuel lines is getting too high, this may mean that a fuel filter is clogged.

Now let's peek inside the train.


Riding the Train

Amtrak interior
A look at the modernized interiors of Amtrak’s bi-level “Superliner” fleet, to be renovated between 2021 and 2024. Amtrak

The U.S.' primary commuter rail network is Amtrak, which covers much of the continental states, as well as a few stops in Canada. Some regional services include Caltrain in California, Atlanta's MARTA, and the Washington Metro around D.C. Amtrak is replacing 40 percent of its trains (some of which are 50 years old) by 2031, with a newer fleet of dual-power trains, which can run on diesel and electricity, as well as adding some diesel-only locomotives for areas of the country with non-electrified rails.

Although taking the train might be slower than flying, it's definitely a lot more comfortable. There is plenty of room to walk around. On Amtrak, dining cars are available with cafe seating and outlets to charge your electronics. During service, they also sell meals, snacks, beverages and coffee. For long-haul passengers, private booths and "sleeper" cars with bedding can be arranged.


On some routes, Amtrak even offers a storage space that can be used to haul a personal vehicle along with you. Compared to airlines, rail travel often offers a more spacious, efficient, and affordable way to cross large swathes of lands.

For more information on diesel locomotives and related topics, check out the links that follow.


Diesel Locomotive FAQ

How does a diesel locomotive work?
When diesel is ignited, it gives power to the pistons connected to an electric generator. The generator then produces energy to supply power to the motors that turn the wheels to run the locomotive.
How many horsepower is a diesel locomotive?
A locomotive's diesel engine is connected to an electric generator that is either DC or AC. In either case, the power produced is around 3,200 horsepower. The generator uses this power to convert it into a massive amount of current, approximately 4,700 amperes.
What is the difference between a hybrid diesel locomotive and a traditional locomotive?
A traditional locomotive simply relies on mechanical energy to drive the locomotive. On the other hand, a modern hybrid diesel locomotive combines both electrical and mechanical energies to give better power output. It consists of massive 12 cylinders connected to a two-stroke diesel engine and some heavy-duty generators and electric motors to increase the power output.
Why are locomotives diesel powered?
In terms of efficiency, diesel engines are more powerful and energy-efficient than gasoline engines. This is because diesel engines work on higher compression ratios. This gives about 20 percent more efficiency than gasoline engines at the same compression ratio.
Why do trains have steel wheel?
Trains have steel wheels to decrease the rolling friction. Tires get compressed after every rotation, which wastes around 25 percent of the engine’s efficiency. So, metal or steel wheels give low rolling resistance, consume less energy and save operating costs.