How a Steam Locomotive Works, From Coal to Wheels

A steam locomotive burns fuel to boil water, then uses the steam's pressure to push pistons that turn the wheels. The whole machine is a boiler on wheels, and nearly every part of it serves one of three jobs: making steam, using steam, or getting rid of the used steam in a way that keeps the fire burning.

Step 1: the firebox and the boiler

Coal is the usual fuel, though coke, oil and, rarely, wood have also been used. It burns on a grate in the firebox. Steamlocomotives.org says the fire can reach over 1,000 degrees Celsius. Hot gases from the fire pass through dozens of fire-tubes that run through a water-filled boiler. Wikipedia explains that the water turns to steam, which takes up roughly 1,600 to 1,700 times the volume of the water it came from.

The boiler typically holds steam at about 200 to 250 pounds per square inch, according to Steamlocomotives.org, and spring-loaded safety valves release any excess. The crew must keep the water level above the top of the firebox. A fusible plug, which melts if the water runs too low, gives a warning.

Step 2: the regulator and superheater

Steam collects at the top of the boiler. The driver opens a regulator (the throttle) to let it out. On most twentieth-century engines it first passes through a superheater, which reheats it so it expands with more force.

Step 3: cylinders and pistons

Steam enters steam chests and is directed by valves into the cylinders, where it pushes a piston first one way and then the other. Because it works on both strokes, the cylinder is called double-acting. A piston rod pushes a crosshead, which pushes a connecting rod that turns the driving wheels. Coupling rods link the driving wheels so that they turn together.

Most engines have two cylinders set a quarter-turn apart, so the engine can always start from a stopped position. Larger engines often had three or four.

Step 4: the valve gear and cut-off

The valve gear times when steam goes in and out of each cylinder. The driver can change how long steam is let in during each stroke, called the cut-off. For starting a heavy train, Steamlocomotives.org says a long cut-off of around 75 percent is used. Wikipedia says about 10 percent can be used for efficient cruising, because the trapped steam keeps expanding and doing work after the supply is shut off.

Step 5: the blastpipe, the sound and the draught

Used steam leaves the cylinders through a nozzle called the blastpipe, pointing up the chimney. The jet drags gases out of the smokebox and pulls fresh air through the fire. The harder the engine works, the more exhaust steam it makes, so the stronger the draught and the more steam it produces. This is also the source of the "chuff" sound.

Crew, tender and injectors

A driver and a fireman are the minimum crew. Larger engines pull a tender holding coal and water. Water is forced into the boiler against its own pressure by injectors, which use a jet of steam. A brake system runs through the whole train and applies automatically if the train parts.

A short history

• 1804: Richard Trevithick's engine hauls a load at Pen-y-darren in South Wales, the first recorded steam-hauled rail journey.
• 1812: Matthew Murray's Salamanca becomes the first commercially successful steam locomotive.
• 1825: Locomotion No. 1 begins hauling passengers on the Stockton and Darlington Railway, the first public steam railway.
• 1829: George and Robert Stephenson's Rocket wins the Rainhill Trials; the Liverpool and Manchester Railway opens in 1830.
• 1934: Flying Scotsman is the first steam locomotive officially recorded at 100 mph (160 km/h).
• 1938: Mallard sets an official record of 126 mph (203 km/h), which has not been broken.

Why steam faded

From the early 1900s, electric and diesel locomotives gradually took over. Water quality, boiler maintenance and fuel costs added to the pressure, according to Wikipedia, and by the 1980s most steam locomotives had been retired from regular service. Many survive on heritage and tourist lines. The largest, the Union Pacific Big Boy, weighs 540 long tons and has a tractive effort of 135,375 pounds-force.

Sources

• Steam locomotive, Wikipedia
• How a Steam Locomotive Works, Steamlocomotives.org