by Chris Woodford. Last updated: January 15, 2022.
Imagine living off nothing but coal and
water and still having enough energy
to run at over 100 mph! That's exactly what a steam locomotive can do.
Although these giant mechanical dinosaurs are now extinct from most of
the world's railroads, steam technology lives on in people's hearts and
locomotives like this still run as tourist attractions on many heritage
Steam locomotives were powered by steam engines, and deserve to be
remembered because they swept the world through the Industrial
Revolution of the 18th and 19th centuries. Steam engines rank with
radio, and television
among the greatest inventions of all time. They are marvels of machinery and excellent
examples of engineering, but under all that smoke and steam, how
exactly do they work?
Photo: A steam-powered railroad locomotive operating at Tweetsie Railroad in North Carolina.
This is a narrow-gauge train, which means the track is not as wide as in a conventional railroad. Narrow tracks
are often used in mountainous areas and in other difficult terrain, because they are generally cheaper to build.
Credit: Photographs in Carol M. Highsmith's America Project in the Carol M. Highsmith Archive,
Library of Congress, Prints and Photographs Division.
What powers a steam engine?
It takes energy to do absolutely anything
you can think of—to ride on a skateboard, to
fly on an airplane, to walk to the shops, or to drive a car down the
street. Most of the energy we use for transportation today comes from
oil, but that wasn't always the case. Until the early 20th century, coal was the
world's favorite fuel and it powered everything from trains and ships
to the ill-fated steam planes invented by American scientist
Samuel P. Langley, an early rival of the Wright brothers. What was so
special about coal? There's lots of it inside Earth, so it was
relatively inexpensive and widely available.
Coal is an organic chemical, which means
it's based on the element
carbon. Coal forms over millions of years when the remains of dead
plants get buried under rocks, squeezed by pressure, and
cooked by Earth's internal heat.
That's why it's called a fossil fuel. Lumps of coal are really lumps of
energy. The carbon inside them is locked to atoms of hydrogen and
oxygen by joints called chemical bonds. When we burn coal on a fire,
the bonds break apart and the energy is released in the form of heat.
Coal contains about half as much energy per kilogram as cleaner fossil fuels such as gasoline, diesel, and kerosene—and that's one reason why steam engines have to burn so much of it.
Photo: The main parts of a steam locomotive.
(For an alternative, side-view, look here.) This is ex-British Railways Standard 4MT tank locomotive number 80104 (built at Brighton in 1955)
working on the Swanage Railway, England in August 2008.
Read how it was restored from a rusting heap and returned to service by
its owners, Southern Locomotives, in
What is a steam engine?
A steam engine is a machine that burns coal to release the heat
energy it contains—so it's an example of what we call a heat engine. It's
a bit like a giant kettle sitting on top of a coal fire. The heat from the fire boils the water in the kettle and turns it into steam. But instead of blowing off uselessly into the air,
like the steam from a kettle, the steam is captured and used to power a
machine. Let's find out how!
How a steam engine works
Crudely speaking, there are four different parts in a steam engine:
- A fire where the coal burns.
- A boiler full of water that the fire heats up to make steam.
- A cylinder and piston, rather like a bicycle pump but much
bigger. Steam from the boiler is piped into the cylinder, causing the
piston to move first one way then the other. This in and out movement
(which is also known as "reciprocating") is used to drive...
- A machine attached to the piston. That could be anything from a
water pump to a factory machine... or even a giant steam locomotive
running up and down a railroad.
That's a very simplified description, of course. In reality, there are hundreds or perhaps even thousands of parts in even
the smallest locomotive.
It's easiest to see how everything works in our little animation
of a steam locomotive, below. Inside the locomotive cab, you load coal
into the firebox (1), which is quite
literally a metal box
containing a roaring coal fire. The fire heats up the boiler—the "giant
kettle" inside the locomotive.
The boiler (2) in a steam locomotive
doesn't look much like
a kettle you'd use to make a cup of tea, but it works
the same way, producing steam under high pressure.
The boiler is a big tank of water with dozens of thin metal tubes
through it (for simplicity, we show only one here, colored orange).
The tubes run from the firebox to the chimney, carrying the heat and
the smoke of the fire with them (shown as white dots inside the tube).
This arrangement of boiler tubes, as they are called, means the
fire can heat the water in the boiler tank much faster, so it produces steam
more quickly and efficiently. The water that makes the steam either
comes from tanks mounted on the side of the locomotive or from a separate wagon called a tender, pulled behind the
locomotive. (The tender also carries the locomotive's supply of coal.) You can see a photo
of a tender showing its water tank further down this page.
The steam generated in the boiler flows down into a cylinder (3)
just ahead of the wheels, pushing a tight-fitting plunger, the piston
(4), back and forth. A little mechanical gate in the cylinder, known as
an inlet valve
(shown in orange) lets the steam in. The piston is connected to one or
more of the locomotive's wheels through a kind of arm-elbow-shoulder
joint called a crank and connecting rod
As the piston pushes, the crank and connecting rod turn the
locomotive's wheels and power the train along (6).
When the piston has reached the end of the cylinder, it can push no
further. The train's momentum (tendency to keep moving) carries the
crank onwards, pushing the piston back into the cylinder the way
it came. The steam inlet valve closes. An outlet valve opens and the
piston pushes the steam back through the cylinder and out up the
locomotive's chimney (7). The intermittent chuff-chuff noise that a
steam engine makes, and its intermittent puffs of smoke, happen when
the piston moves back and forth in the cylinder.
There's a cylinder on each side of the locomotive and the two cylinders
fire slightly out of step with one another to ensure there's always some
power pushing the engine along.
Types of steam engine
Photo: Close-up of the piston and cylinder in a steam engine.
Our diagram up above shows a very simple, one-cylinder steam engine powering
a steam locomotive down a track. This is called a rotary
engine, because the piston's job is to make a wheel rotate. The
earliest steam engines worked in an entirely different way. Instead of
turning a wheel, the piston pushed a beam up and down in a simple
back-and-forth or reciprocating motion.
engines were used to pump water out of flooded coal mines in the early
Our diagram shows steam pushing the piston one way and the momentum
of the locomotive driving it the other way. This is called a single-acting
steam engine and it's quite an inefficient design because the piston is
being powered only half the time. A much better (though slightly more
complex) design uses extra steam pipes and valves to make steam push
the piston first one way and then the other. This is called a double-acting
(or counterflow) steam engine.
It's more powerful because steam is driving the piston all the
Animation: In a double-acting cylinder, a valve (orange) flips back and forth allowing steam to enter (yellow) and exit (red) the cylinder from both directions, so providing power twice as much of the time. I've simplified the mechanism here so it's easy to understand. The valve actually slides from side to side rather than flipping over.
If you look closely at the wheels of a typical steam engine, you'll
see that everything is more complex than we've seen in the simple animation up above:
there's much more machinery than just a single crank and connecting rod. In fact, there's an
intricate collection of shiny levers, sliding back and forth with meticulous
precision. This is called the valve gear. Its job
is to open and close the cylinder valves at just the right moments to let
steam in from either end, both to make the engine work as efficiently and powerfully as possible and to allow it to
drive in reverse. There are quite a few different types of
valve gear; one of the most common designs is called the Walschaerts, named for
its Belgian inventor Egide Walschaerts (1820–1901). The tank engine 80104
shown in the second photo on this page has a Walschaerts-type valve gear, and so does
Eddystone, the locomotive pictured below.
Photo: The Walschaerts valve gear on a typical large steam locomotive,
The first steam engines were very large and inefficient, which means
it took huge amounts of coal to get them to do anything. Later engines
produced steam at much higher pressure: the steam was produced in a
smaller, much stronger boiler so it squeezed out with more force and
blew the piston harder. The extra force of high-pressure
engines allowed engineers to make them lighter and more compact,
and it was this that paved the way for steam locomotives, steam ships,
and steam cars.
Photo: Steam engines could not carry all the water
they needed for a long journey. Periodically, they would have to stop to refill at
track-side water tanks like this one (above) on the Swanage Railway.
Larger engines had tenders: trucks they hauled behind that held supplies of
coal (in front of the red line we've drawn) and water (behind the red line). The coal rests on an angled
plate inside the tender that makes it tip naturally toward an opening
at the front where the fireman can easily shovel it into the firebox.
Below: You can see what the tender's like inside on this unusual photo of an empty tender,
photographed from slightly above and behind, taken at Think Tank, the museum of science in Birmingham, England. This tender holds about 18000 liters (4000 UK gallons) of water and belongs to the museum's City of Birmingham locomotive.
Did steam really die?
Coal was a cheap and abundant fuel during the early Industrial
Revolution, but the invention of the gasoline engine
(petrol engine) in the mid-19th century heralded a new era:
during the 20th century, oil overtook coal as the world's favorite
fuel. Steam engines are extremely inefficient, wasting around 80–90 percent
of all the energy they produce from coal. That means they have to burn
enormous amounts of coal to produce useful amounts of power.
A steam engine is so inefficient because the fire that burns the coal is
totally separate (and often some distance from) the cylinder that turns
the heat energy in the steam into mechanical energy that powers the
machine. This design is called an external combustion engine
because the fire and boiler are outside the cylinder. It's inefficient
because energy is wasted as the heat and steam travel from the fire,
via the boiler, to the cylinder. Gasoline- and diesel-powered engines are based on a totally different design called
an internal combustion engine. The gasoline or diesel fuel
is burned inside the cylinder, not outside it, and this makes
internal combustion engines considerably more efficient.
(You can read more about internal and external combustion in our overview
Oil has many other advantages too: it's cleaner than coal, makes less
air pollution, and is much easier to transport in pipes.
That's largely why steam locomotives disappeared from our railroads—diesel locomotives were
altogether more convenient. It takes hours to fire up a steam engine before you can use it; you can
get a diesel engine running in less than a minute. Steam engines disappeared from factories when electricity
became a more convenient way of powering buildings. Who wants to load coal into a factory every day when they can just
flick on switches to make things work?
Artwork: Less is more: The UK switched from steam engines to diesel and electric during the 1960s. The last engines were built there in 1956 and the very last steam train ran in August 1968. By 1968, there were only about a third as many locomotives in service as there had been in 1962, but just as much freight was being hauled: the diesel-electric rail system was apparently
much more efficient. Source: Drawn using data from "The Performance of British Railways 1962–1968" by C.D.Jones, Journal of Transport Economics and Policy, Vol. 4, No. 2 (May, 1970), pp. 162–170.
But things are not quite what they seem. Steam and coal never did
Where does the electricity we use come from?
It would be great if it all came from renewable energy
(wind turbines, solar panels, and so on), but
much of it still comes from coal,
burned in power plants miles away from
our homes and factories.
Inside a coal-fired power plant, coal is still burned to make steam, driving windmill-like devices called
steam turbines, which are much more efficient than steam engines. As they rotate, they turn
electromagnetic generators and produce electricity.
So, you see, although steam locomotives have vanished from our
railways, steam power
is alive and well—and just as important as it ever was!
Photo: Some of the steam engines that run on heritage lines
were still relatively new when they were withdrawn from service.
Bulleid Pacific No. 34070 "Manston,"
was built in 1947 and withdrawn less than 20 years later (in 1964).
After a long restoration by Southern Locomotives, it returned to
service on the Swanage Railway in September 2008.
A wonderfully impressive sight, it weighs 128 tons and can reach speeds of over 160km/h (100mph).
Who invented the steam engine... and when?
Here's a brief history of steam power:
- 1st century CE: Hero of Alexandria
demonstrates a steam-powered spinning sphere called an aeolipile.
- 16th century CE: Italian architect Giovanni
(1571–1640) uses a steam jet to rotate the blades of a small wheel,
anticipating the steam turbine developed by Sir Charles Parsons in 1884.
- 1680: Dutch physicist Christiaan Huygens
makes the first piston engine using a simple cylinder and piston
powered by exploding gunpowder. Huygens' assistant Denis
(1648–c.1712) realizes steam is a better way to drive a cylinder and
- 1698: Thomas Savery (c.1650–1715)
steam-powered water pump called the Miner's Friend. It's a simple
reciprocating steam engine (or beam engine) for pumping water from
- 1712: Englishman Thomas Newcomen
(1663–1729) develops a
much better design of steam-powered, water-pumping engine than Savery
and is usually credited with inventing the steam engine. A
Scottish engineer named James Watt
(1736–1819) figures out a
much more efficient way of making power from steam after improving a
model of the Newcomen engine. Watt's improvements of Newcomen's
engine lead to the widespread adoption of steam.
- 1770: French army officer Nicolas-Joseph
(1725–1804) invents a steam-powered, three-wheeled tractor.
- 1797: English mining engineer Richard
(1771–1833) develops a high-pressure steam version of Watt's engine,
paving the way for steam locomotives.
- 1803: English engineer Arthur Woolf
(1776–1837) makes a
steam engine with more than one cylinder.
- 1804: American industrialist Oliver Evans
invents a steam-powered passenger vehicle. Like Trevithick, he
recognizes the importance of high-pressure steam and builds more than
50 steam-powered vehicles.
- 1807: American engineer Robert Fulton
the first steamboat service along the Hudson River.
- 1819: Steam-powered ocean ship Savannah
Atlantic from New York to Liverpool in only 27 days.
- 1825: English engineer George Stephenson
(1781–1848) builds the world's first steam railway between the
towns of Stockton and Darlington. To begin with, steam locomotives pull
only heavy coal trucks, while passengers are ferried in horse-drawn carriages.
- 1830: The Liverpool and Manchester Railway becomes the first to use steam power
for hauling both passengers and freight.
- 1882: The prolific American inventor Thomas
(1847–1931) opens the world's first commercial power plant at Pearl
Street, New York. It uses high-speed steam engines to power the
- 1884: English engineer Sir Charles Parsons
develops the steam turbine for his high-speed steam boat Turbinia.
Photo: Think of steam engines and you probably think of steam locomotives, but ships were steam powered too before diesel engines came along. This one is the beautifully restored PS Waverley, the last ocean-going paddle steamer in the world, dating from 1947 and steaming into Swanage Pier in September 2009.