How a Steam Locomotive Works

Cup of coffee.

Steam is that wispy stuff that comes off your coffee in the morning. It’s the stuff that froths your cappuccino. It is what sets the whistle going on a tea kettle. And it can propel a 100 ton machine forward at over 120 miles per hour.

How does that happen? How does that wisp of steam off your coffee turn into something powerful enough to make the industrial age possible?

Before we had steam locomotives, we had the steam engine. What’s the difference? A steam engine can be stationary and power all sorts of machines. A steam locomotive uses a steam engine on wheels to power itself into motion.

Steam engines multiplied the work one person could do in a day. Or made possible the previously impossible. The steam engine made the water wheel obsolete and allowed more grain to be milled. One of the first things steam engines were used for was to pump water, bringing water to parched communities, or keeping deep mines dry.

Just like steam locomotives which grew larger and larger, stationary steam engines did so too. Here at, we see locomotives as those steam giants, however at the Brede Steam Giants Museum in Sussex, England, UK, the giants are stationary steam pumps. You can read about the museum’s preserved 3-story steam engines here.

How Do Steam Engines and Locomotives Work?

How does our big tea kettle turns steam into energy, motion and force. This animation explains the basic concept of a steam engine.

Steam Engine Animation

As shown in our animation, steam engines work by transforming the potential energy stored in fuel, coal in our example, into kinetic energy. Think of it like a complicated tea kettle consisting of four parts:

  • A firebox full of fuel that bring temperatures to 1,000 degrees Fahrenheit.
  • A boiler full of water that our firebox heat transforms into steam.
  • A system of pipes, valves and regulators that puts the steam under pressure and delivers it to a piston.
  • Cylinders and pistons that our pressurized steam expands pushes against, creating physical motion – the kinetic energy. This back-and-forth motion creates the force we need to drive any equipment.
  • Steam: When we heat water past its boiling point the water turns into a gas, a water vapor or as what we call steam.
  • Engine: Anything that converts energy into a mechanical motion or force. Any fuel can be used by an engine to create the mechanical motion. Some of the first engines were water wheels. As water was directed over the wheel to fall to a basin below, the force of the water turns the wheel. We’ve taken energy, the gravity of the falling water, and turned it into a motion that can be used to power machines.
  • Steam engine: An engine that uses steam, specifically steam under pressure, to create a force.
  • Steam locomotive: A machine that uses a steam engine on wheels to propel itself.

Now imagine that steam engine in our animation tipped over to be horizontal, and add wheels. We’re on on way to a steam locomotive! The pressurized flow of steam forces the locomotive’s inner mechanics to turn. The continuous transformation of energy propels the train forward. However, it took awhile to get from a simple steam engine to the first successful steam locomotive.

It’s generally acknowledged the first known steam engine was built in the first century in Alexandria, Egypt. In the early 16th century, Italian engineer Giovanni Branca used steam to rotate blades of a wheel, foreshadowing the invention of a steam-powered turbine engine.

The Savery and Newcomen steam engines. Two early steam engines. Both used to power a pump water.
On the left, the Savery Engine. On the right, the Newcomen Engine.

In 1698 Thomas Savery invented the “miner’s friend”, a water pump powered by a reciprocating steam engine. Fourteen years later, Thomas Newcomen created a better design, leading to a more powerful steam-powered water pump. For his invention, Newcomen is often created as the father of the steam engine, however his initial design would later be improved upon. Scottish engineer James Watt in particular contributed important innovations such as separate chambers for heated and cooling steam, leading to a more efficient engine.

Improvements over the next century allowed the steam engine to power boats and ships and tractors, but the locomotive that could pull freight – that would have to wait. Many inventors would work on the problem, but George Stephenson is credited with the first successful steam locomotive. In 1825 the Stockton & Darlington Railway used Stephenson’s locomotive to help bring coal from interior mines to the coast for shipping.

Although the first steam locomotive wasn’t invented until the 19th century, a long line of inventions paved the way. Exactly how a steam locomotive works changed over time. for this fantastic creation. Here’s a brief overview of all the innovations that lead to the creation of the steam locomotive:

1st Century AD – Hero of Alexandria invents the Aeolipile, a steam-powered engine built from spinning spheres.

Early 16th Century AD – Giovanni Branca rotates the blades of a wheel using steam, foreshadowing the invention of a steam-powered turbine engine.

  • 1680 – Christiaan Huygens invents the first single-cylinder engine using a cylinder, piston, and some gunpowder. Huygens’ co-invent, Denis Papin, suggests using steam rather than gunpowder.
  • 1698 – Thomas Savery invents the miner’s friend, a water pump that runs off a reciprocating steam engine.
  • 1712 – Thomas Newcomen creates a more powerful, better-designed steam-powered water pump. For his invention, Newcomen is often created as the father of the steam engine however his initial design would later be improved upon by both Scottish and French engineers.
  • 1770 – Nicolas-Joseph Cugnot powers a three-wheeled tractor using a steam engine for the first time
  • 1797 – Richard Trevithick invents the first high-pressure steam engine,
  • 1803 – Arthur Woolf invents the first high-pressure steam engine with multiple cylinders, thus increasing the amount of power a steam engine can create.
  • 1804 – Oliver Evans invents the first passenger vehicle operated using a high-pressure steam engine.
  • 1807 – Robert Fulton invents the first high-pressure steamboat and sails down the Hudson River.
  • 1819 – Savannah, the first steam-powered ocean liner, creases from New York to Liverpool, breaking the record for the fastest trans-Atlantic crossing of its time.
  • 1825 – George Stephenson invents the first steam-powered locomotive and travels from Darlington to Stockton, England.
  • 1865 – James Watt invents the condenser. The condenser chamber steam to be cooled which allows the resulting vacuum to allow two “powered” strokes to a piston. This efficiency results in 60% less fuel needed for the same work produced by a “Newcomen” engine.

Once industrialists realized how quickly steam locomotives could transport goods across the English countryside, they set to work to improve and strengthen the steam locomotive. In just a few decades rail lines covered England and steam locomotives were getting larger and faster.

But we’re ahead of ourselves. How did George Stephenson and later engineers tip that steam engine over and give it enough energy to propel itself and pull a train of cars? Steam might not seem like much but put it under pressure and it can do a lot – even move a massive train. In this animation, note how similar the technology is to the stationary steam engine above. The difference is the improvement in how we harness that steam pressure. Let’s look at how a steam locomotive works.

Steam Locomotive Animation

Everything Works Together to Create Motion

All the moving parts in a steam locomotive are responsible for transforming potential energy stored in a fuel (like coal in our example) into rotational kinetic energy – turning a wheel. It takes a lot of parts to turn steam into motion but we’ll do our best to break down each of the moving parts and explain their role in how a steam locomotive works.

  • The firebox – The firebox is a heavy metal box designed to hold coals burning at temperatures as high as 1,000 degrees Fahrenheit. The firebox sits below a boiler and releases potential energy stored in coals as heat.
  • The boiler – The boiler is essentially just a giant water kettle. Although much larger, it stores water and heats up as the firebox burns below it. Boilers are divided by small tubes that carry heat and smoke up to a chimney and as heat travels upwards, it quickly boils the water stored within. The boiler is in turn supplied by tanks of water mounted to the side of the train or from a separate tender.
  • Cylinders – As the water boils into steam in the boiler, the building pressure forces the steam into a series of cylinders ahead of the wheels and presses against a series of pistons.
  • Pistons – Pistons are reciprocating bits of metal that move back and forth within a cylinder. As a small inlet gate lets steam into the cylinder, it moves the piston up and down, creating kinetic energy.
  • Crankshaft and connecting rod – As the pistons start moving, they push a crankshaft which transforms their reciprocating motion into rotational kinetic energy. The crankshaft connects to a connecting road which then transfers the rotational energy into the wheels. See below.
Piston, Rods, Linkages Animation
How a Steam Locomotive Works. Here the pistons move the rods and linkages that move the wheels.
Walschaerts valve gear. See KEY below. (Animation by Panther. CC BY-SA 3.0)

In the animation above, Item 2 is a crank outside of the main crankpin. This provides the main force to move the valve. Item 1 is connected to a pushrod, Item 8, controlled by the engineer. Pushing the control rod causes linkage 3 to move up and down. If the link is below center, the engine moves backwards. If the link is near to the center, steam is conserved for ordinary running. Item 4 guides the piston rod. Item 5 is the pivot where the main connecting rod connects to the crosshead an receives the motion transferred from the piston. Items 6 and 7 are the cylinder (7) and valve block (6). Pink is steam from the boiler. Exhaust is white.

Putting all the Parts Together

When you first look at a steam locomotive, you instantly notice rods attached to the wheels. As a train moves forward, the rods rotate with the wheels. Children even imitate this motion by bending their arms and gyrating them in a circle. It’s these rods that propel the locomotive.

Coupling rods link the driving wheel with all the other wheels on the train. Standard steam locomotives only have two cylinders—one on either side of the engine. This means that only one wheel receives energy directly from the crankshaft. It’s up to the coupling rods to transfer energy from the driving wheel through the other wheels on each side of the train.

Coupling rods on the Big Boy steam locomotive. It's one of the many parts needed to make a steam locomotive work.
The massive coupling or side rods of a Union Pacific Big Boy Steam Locomotive.

Coupling rods help make steam engines more efficient so the engine doesn’t require multiple pistons. The engineer of a steam locomotive is tasked with running the engine efficiently. Running a steam locomotive efferently is not only cost effective, as it uses less fuel, but ensures the locomotive won’t run out of water or fuel before it reaches its next water and fuel top-off.

While early steam locomotives were inefficient and required massive amounts of coal and water to move anything, later designs could withstand higher steam pressure and move hefty loads without burning as much coal. That made it possible for locomotives to become larger and more powerful.

For the most part, all steam locomotives work on the same principle – a fuel heats water, water becomes steam, steam drives a piston that turns the crankshaft. However, in a more efficient locomotive, the piston isn’t moved back into place by the crankshaft. Instead, the steam pushes the piston back into place thanks to a series of sliding levers and a valve gear that regulates when the cylinder opens and closes.  

By using steam to push the piston back into place, steam locomotive engines can run far more efficiently and with more power. Furthermore, trains can move in reverse. But this wasn’t the final iteration of the steam engine. The valve gear underwent a series of further improvements before many railroads and manufacturers settled on one of the most common designs, the Walschaerts (see animation above), named for its inventor Egide Walschaerts.

Where have all our steam locomotives gone?

The earliest steam locomotives used wood for fuel. That quickly moved to coal which was both cheap and plentiful. Coal and occasionally fuel oil, burned in the firebox remained the best way to create steam for a century. Halfway through that century inventors began to work with oil and cracking it into diesel fuel and then gasoline.

Although these proud C&NW Railroad steeds may not have know it, by 1942 the diesel locomotive revolution had begun.
Chicago & North Western roundhouse. By this time, 1942, diesels had just begun to take over. Photo: Jack Delano

Just as the steam engine had to be improved over years to make it commercially viable, so did the new internal combustion engine. combustion engines don’t lose heat in the same way a firebox (external combustion) does. The majority of the energy released from burning diesel or gasoline can go directly into powering an engine whereas much of the heat released from coal is lost to the surrounding environment.

By the 1950s, most everyone could hear the bell tolling for the steam locomotive. The internal combustion engine has fewer moving parts, is more efficient and easier to control. All that means much less maintenance and a longer life-span. These are things railroad companies needed to stay competitive.

Rather quickly the diesel locomotive replaced the steam locomotive on railroads. In the United States the process was more-or-less complete by 1965. Most of the rest of the world had dieselized, or electrified, their railroads by 1980. A few small operations or poorer countries carried on for a couple of decades. There is only one place in the world that still uses big steam locomotives for freight service on a daily basis, in Bosnia-Herzegovina. For that story, see our article, “Amazing! Steam in Freight Service in 2022“.

Santa Fe 3751 conquers Tehachapi Loop. (From the Pentrex show, “3751 Return to Steam“.

Steam afficadoes however can thank an army of volunteers for saving hundreds of steam locomotives from the scrappers torch, some of which continue to operate today on museum and tourist lines around the world.

So, no more steam engines?

While steam locomotives are no longer used in daily service by the world’s railroads, that doesn’t mean that steam engines are not used. Most nuclear power plants use steam engines to generate power. The fuel is not coal, of course, but nuclear rods.

Most of the electricity that powers our homes is made from steam engines. However, rather than using pistons and cylinders, powerplants use steam to spin a massive electromagnet. As the magnet spins, it creates electricity.

There are solar thermal power plants that use the sun to heat water and use the steam generate electricity via a turbine. Steam engines, the burning of fuel to heat water into steam that transforms the energy in the fuel into kinetic energy, are still used in many applications today.

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