Steam locomotive

• 1. Introduction

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  Heat water in a steam locomotive's boiler until it turns to gas, and it swells to somewhere between 1,600 and 1,700 times its original volume. That single expansion is the secret of the steam locomotive. It is a steam engine that carries itself on wheels, and it hauled the world's trains for roughly a century and a half.

  How did a handful of experimenters in mines and ironworks learn to harness that swelling? What really happens in the firebox, the cylinders and the cab when a locomotive pulls away? What did these engines have to eat and drink, and what did crews do when the supply ran dry? And why did a machine that once seemed unbeatable end up mostly in museums and on tourist lines?

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• 2. Collieries to Rainhill

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  The earliest railways relied on horses to draw carts along the rails. In 1784 a Scottish inventor named William Murdoch built a small prototype of a steam road locomotive in Birmingham. Around 1787 William Reynolds proposed a full-scale rail locomotive, but experts dispute whether it was real and when it dates from.

  A workable steam train had to wait for Richard Trevithick and his high-pressure steam engine. In 1802 he built the Coalbrookdale Locomotive for the ironworks of that name in Shropshire. No record survives of it ever working there.

  On the 21st of February 1804, another Trevithick locomotive hauled a train along the tramway from the Pen-y-darren ironworks, near Merthyr Tydfil, to Abercynon in South Wales. Andrew Vivian rode with it, and the run met with mixed success. High-pressure steam let the engine be lighter and more efficient.

  Later in 1804 Trevithick visited the Newcastle area and found colliery owners and engineers eager to listen. The visit went so well that the colliery railways of north-east England became the leading centre for locomotive experiment. Trevithick himself carried on with three more locomotives. The last, Catch Me Who Can in 1808, was the first in the world to haul fare-paying passengers.

  In 1812 Matthew Murray's Salamanca made its first run on the Middleton Railway. It had twin cylinders and ran by rack and pinion on edge rails, and it became the first commercially successful steam locomotive. Puffing Billy, built in 1813-14 by the engineer William Hedley for the Wylam Colliery near Newcastle upon Tyne, is the oldest locomotive still preserved. It sits on static display at the Science Museum in London.

  George Stephenson, a former miner working as an engine-wright at Killingworth Colliery, built as many as sixteen Killingworth locomotives. Blücher came in 1814 and Killingworth Billy in 1816. In 1817 he built The Duke for the Kilmarnock and Troon Railway, the first steam locomotive to work in Scotland.

  In 1825 Stephenson's Locomotion No. 1 went into service on the Stockton and Darlington Railway in north-east England, the first public steam railway anywhere. Four years later his son Robert built The Rocket in Newcastle and entered it in the Rainhill Trials, which it won. The trials' promoters insisted that every contender have a proper way to read boiler pressure. Stephenson answered with a nine-foot vertical tube of mercury, topped by a sight-glass and mounted beside the chimney.

  That win made Robert Stephenson and Company the leading locomotive builder for railways in the United Kingdom, the United States and much of Europe. The Liverpool and Manchester Railway opened the following year, and it ran its passenger and goods trains on steam power alone.

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• 3. Rails Beyond Britain

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  In 1794 the steamboat pioneer John Fitch designed and built an early working model of a steam rail locomotive in the United States. Some sources say it already worked in the 1780s and that he showed it to George Washington. The model survives at the Ohio Historical Society Museum in Columbus.

  In 1825 Colonel John Stevens demonstrated a full-sized "steam wagon" on a loop of track in Hoboken, New Jersey. Commercial American railroads, though, began with British imports, first the Stourbridge Lion and then the John Bull. The original John Bull was on static display at the National Museum of American History in Washington, D.C., as of 2021. A replica is kept at the Railroad Museum of Pennsylvania.

  A domestic industry followed quickly. Peter Cooper's Tom Thumb ran on the Baltimore and Ohio Railroad in 1830 as the first commercial US-built locomotive. It was meant to show what steam traction could do rather than to earn revenue. The DeWitt Clinton, built in 1831 for the Mohawk and Hudson Railroad, was another notable early engine.

  Germany's first working steam locomotive was built in June 1816 by Johann Friedrich Krigar at the Royal Berlin Iron Foundry. It followed the rack-and-pinion system of John Blenkinsop, the same idea as Salamanca. It circled a track in the factory yard, and curious visitors could pay to ride in its coaches. That made it the first locomotive built on the European mainland. A New Year's badge from the foundry, dated 1816, shows it.

  A second locomotive on the same system followed in 1817. Both were meant for pit railways at Königshütte and at Luisenthal on the Saar, but after being taken apart, moved and rebuilt, neither could be made to run again.

  On the 7th of December 1835 the Adler made its first run between Nuremberg and Fürth on the Bavarian Ludwig Railway. It was the 118th engine from Robert Stephenson's works and was protected by patent. Germany's third locomotive, the Saxonia, came from Professor Johann Andreas Schubert near Dresden in 1838. The first independently designed German engine was August Borsig's Beuth in 1841. Henschel-Werke in Kassel delivered its first, the Drache, in 1848.

  In Russia, the Cherepanovs built a locomotive in 1834. It suffered from a lack of coal in the area, and horses took over once the nearby woods had been cut down. The Tsarskoye Selo steam railway began in 1837 with locomotives bought from Robert Stephenson and Company. That same year Austria's first steam railway opened on the Emperor Ferdinand Northern Railway, from Vienna-Floridsdorf to Deutsch-Wagram.

  Italy's first steam locomotives, Bayard and Vesuvio, ran on the Napoli-Portici line in the Kingdom of the Two Sicilies. Swiss territory saw its first railway with the Strasbourg-Basel line in 1844. Three years later the Spanisch Brötli Bahn, from Zürich to Baden, became the first fully Swiss line.

  The oldest continually working steam engine in the world still runs in Austria. GKB 671, built in 1860, has never left service and still pulls special excursions.

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• 4. Fire and Draught

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  Around 1,000 locomotives in Hungary used the water-tube Brotan boiler, which makes them the great exception. Nearly everywhere else the fire-tube boiler was standard. Nigel Gresley tested a water-tube design on the LNER Class W1 in search of better thermal efficiency, but the difficulties outweighed the appeal.

  The standard boiler has three parts: a firebox where the fuel burns, a barrel where water becomes steam, and a smokebox. A fireman throws solid fuel through a door onto the grate, and ash drops into an ashpan. Tubes run from the firebox to the smokebox, and hot gases flowing through them hand their heat to the surrounding water. Water also surrounds the firebox itself, to keep its metal from overheating.

  The famous chuffing sound comes from the exhaust. Used steam from the cylinders shoots through a nozzle up the chimney and drags the smokebox gases along with it. That keeps pressure in the smokebox lower than beneath the grate, so air rushes up through the burning coal bed. Because the draught depends on the exhaust, the fire automatically strengthens when the engine works harder.

  Shaping the exhaust became a science of its own. Engineers such as Chapelon, Giesl and Porta made large gains in efficiency and cut maintenance time and pollution. A draught that was too strong could lift the fire off the grate or throw out unburnt fuel, which gave steam locomotives their unenviable reputation for dirt.

  Water level was the crew's constant worry. A transparent sight glass shows it, and the level must stay between two marked lines. If it drops too low, the crown sheet at the top of the firebox is exposed. With no water to carry the heat away, it softens and fails, letting high-pressure steam into the firebox and cab. The fusible plug, a temperature-sensitive device, was developed to vent steam in a controlled way and warn the fireman.

  Before the 1870s many engines used try-cocks instead. At least two, and usually three, were fitted at different heights, and the crew opened each to see whether steam or water came out. As pressures climbed they grew dangerous, and scale or sediment could block them and give false readings. The sight glass replaced them.

  Safety valves had their own history. The earliest used a weight hanging from a lever, which bounced on rough track and wasted steam. A spring column made by Salter, a well-known maker of spring scales, replaced it. Crews could add weight to the arm to raise pressure, so John Ramsbottom introduced a tamper-proof valve in the late 1850s. Tampering with it could only ease the pressure.

  George Richardson's American valve of 1875 opens only when the pressure reaches its permitted maximum, and it is in almost universal use today. Britain's Great Western Railway kept the older direct-loaded type until its separate existence ended, believing it lost less pressure between opening and closing. The Bourdon tube gauge, introduced in 1849, replaced the guesswork of reading a valve arm.

  The word lagging for boiler insulation comes from the cooper's term for a barrel stave. Salamanca and Locomotion No. 1 both wore wooden lagging. In the later days of steam, stitched mattresses stuffed with asbestos took over, and most countries have since banned the material for health reasons. Modern practice uses glass wool or aluminium foil, and under L. D. Porta's influence even cylinder ends are insulated to cut warm-up time.

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• 5. From Piston to Wheel

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  A two-cylinder locomotive has one cylinder on each side, and their cranks are set 90 degrees out of phase. At the two dead centres, a connecting rod lines up with its crankpin and delivers no torque. The offset guarantees both sides are never stuck at once. Each wheel revolution brings four power strokes, because steam enters each end of each cylinder in turn.

  The valve gear controls that admission and also reverses the engine. Its key setting is the cut-off, which decides how much of the stroke receives steam. A 50% cut-off admits steam for half the stroke and lets expansion do the rest. The reversing lever, called the Johnson bar in the US, therefore works much like a car's gearshift. Full cut-off pulls away from a standstill, while cruising can use a cut-off as low as 10%, saving fuel and water.

  Early locomotives had a simple valve gear that gave full power in either direction. Stephenson valve gear soon let drivers control the cut-off, and Walschaerts gear and similar designs largely replaced it. The Stephenson type sat inside the frame and was hard to reach for maintenance.

  The wheels themselves posed a problem of grip. Power reaches the rail from zero rpm upward, so the driving wheels must adhere to a smooth surface. In North America, equalising levers between axles kept wheel loads even on uneven track. Engines with all wheels coupled were unstable at speed, so builders added unpowered carrying wheels on trucks or bogies to guide them through curves.

  The wheel arrangement told a story. A 4-4-2, known in America as the Atlantic type, had a trailing axle that could support a huge firebox. Most were called free steamers, able to hold steam pressure whatever the throttle setting. Two driving axles carried less reciprocating mass than three or more, so such engines could turn very fast.

  Cylinder layouts varied widely. Three or four cylinders gave a smoother power cycle and more output, at the cost of more complicated gear and upkeep. Most British express engines built between 1930 and 1950 used them. From 1951, though, all but one of the 999 new British Rail standard steam locomotives had just two cylinders for easier maintenance.

  Compound locomotives, in use from 1876, expanded steam twice or more through separate cylinders to reduce heat lost to cylinder cooling. André Chapelon's rebuilds from 1929 showed a dramatic jump in power. Anatole Mallet's compound articulated design fixed the high-pressure engine to the boiler frame and pivoted a low-pressure one in front of it.

  Tight curves on narrow-gauge lines called for articulated engines, and the Mallet and the Garratt were the most popular. The Fairlie had two boilers back to back on one frame, and a few triplex designs added a third engine unit under the tender. Duplex locomotives fared poorly. The Pennsylvania Railroad's 4-4-4-4 class T1, built for very fast running, suffered recurring slippage that was never cured.

  Where slow speed and high starting torque mattered, as on logging, mine and quarry lines, geared engines such as the Shay, the Climax and the Heisler were built. Southern Pacific went further with cab forward locomotives, which put the cab and firebox at the front and the tender behind the smokebox, so the engine seemed to run backwards. Oil firing made that possible, and it gave the driver air free of smoke.

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• 6. Fuel and Water

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  Until 1870 most locomotives in the United States burned wood, but as eastern forests were cleared, coal gradually became dominant worldwide. Plentiful coal made it the obvious choice in the UK and elsewhere in Europe from the start. Railways serving sugar cane operations burned bagasse, a byproduct of sugar refining. After 1900, cheap oil won over the southwestern US railroads, particularly the Southern Pacific. Many steam locomotives in the Australian state of Victoria were converted to heavy oil after World War II.

  Some fuel experiments were stranger. German, Russian, Australian and British railways tried firing locomotives with coal dust. During World War 2, Switzerland had a coal shortage but plenty of hydroelectricity, so a number of its steam shunting locomotives were converted to electrically heated boilers. They drew around 480 kW from an overhead line through a pantograph. They were far less efficient than true electric engines, but the country used what it had. Today some heritage engines, including Grand Canyon Railway 4960 and the Disneyland Railroad locomotives, run on used cooking oil.

  On Australia's Trans-Australian Railway, water was the greater burden. The line never crosses a permanent freshwater watercourse, so locomotives used bore water rich in magnesium chloride. No cheap treatment existed, and boilers lasted less than a quarter of their normal lifespan. In the steam era about half of a train's total load there was water for the engine. The operator, Commonwealth Railways, became an early adopter of the diesel-electric locomotive.

  The root of the problem is that a steam locomotive runs on an open cycle. It expels its steam after one use rather than recycling it as stationary and marine engines do. Water towers fed cranes and gantries at stops and depots. In the UK, the US and France, some main lines had water troughs, called track pans in the US. The fireman lowered a scoop under the tender, the speed of the train forced water up into the tank, and the scoop was raised again.

  In the early American West, small communities often lacked refilling facilities, and crews stopped beside a stream to fill the tender with leather buckets. This was called "jerking water", and it gave rise to the term "jerkwater towns" for small settlements. In drier parts of Australia and South Africa, engines used oversized tenders and sometimes an extra water wagon, nicknamed a "water gin" in New South Wales.

  Condensing engines were the desert answer. Their tenders held huge radiators, and exhaust steam was sent back to the tender and condensed instead of leaving through the funnel. Class 25 locomotives, nicknamed the "puffers which never puff", crossed South Africa's Karoo desert from the 1950s until the 1980s. London's Metropolitan Railway fitted condensing apparatus to keep steam out of its tunnels, and such engines were still working between King's Cross and Moorgate in the early 1960s.

  Water quality caused trouble of its own. As boiler pressures rose, dissolved solids made tough-skinned bubbles that could carry water into the steam pipes and blow off cylinder heads. Crews had to blow down hot, mineral-heavy water from the boiler regularly. Swengel, an engineer quoted in the source, suggested these troubles added to interest in electrifying railways.

  In the 1970s L. D. Porta developed a heavy-duty chemical treatment that keeps a boiler clean and prevents corrosion. It also makes the foam form a compact blanket on the water surface that filters the steam as it is produced.

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• 7. The Footplate

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  A crew of at least two normally ran a steam locomotive. The driver, called the engineer in North America, controlled starting, stopping and speed, while the fireman kept the fire going, regulated steam pressure and watched water levels. In the early 20th century some locomotives grew so large that no fireman could shovel fast enough. Steam-powered mechanical stokers became standard in the United States and were also used in Australia and South Africa.

  The oldest lubrication method was surprisingly simple: the enginemen dropped a lump of tallow down the blast pipe. Tallow clings to cylinder walls and resists water better than mineral oil, and it remains an ingredient in modern steam cylinder oil. Frame bearings were fed by capillary action, with trimmings of worsted yarn trailing from oil reservoirs. In British practice, a capsule of aniseed or garlic oil in the bearing metal gave a pungent warning if lubrication failed.

  The blower guarded against a deadly hazard. When the regulator is closed, no exhaust steam draws the fire, so a ring of steam nozzles at the chimney base keeps the draught going. If the driver closes the regulator before opening the blower, air can blow down the chimney, and the fire itself can be thrown back through the firehole onto the footplate. Tunnels raise the risk because of the pressure shock.

  British records show how real the danger was. A 1955 report on an accident near Dunstable counted twenty-three such cases in 1953, injuring 26 enginemen. The year 1954 saw the same numbers plus one death. The problem has not vanished, as shown by a 2012 incident involving BR Standard Class 7 70013 Oliver Cromwell. The firedoor gave some protection against these blowbacks, along with the job of directing air through the fire.

  At the front end, American locomotives carried a plough-shaped pilot, nicknamed the cow catcher, to clear cattle, bison and fallen limbs from the track. European railways and New South Wales mostly considered them unnecessary. Switching engines swapped them for small footboards.

  Britain declined to adopt bright headlights, which could mask the dim oil lamps used on signals and at the rear of trains. Instead, low-intensity lamps on the front told staff what class of train was coming. Four lamp irons were provided, one below the chimney and three across the buffer beam. The Southern Railway added a lamp iron at each side of the smokebox, so that lamp patterns or white plates could tell staff a train's origin and destination.

  Bells warned of moving trains in the United States, India and Canada, and automatic bell ringers came into widespread use in the US after 1910. Only a minority of British locomotives carried bells, because British lines were fenced by law. In Germany and Britain, operating companies began fitting Automatic Warning System equipment early in the 20th century. It applied the brakes automatically when a driver passed a signal at caution, and it became mandatory in Britain in 1956.

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• 8. Records and Retreat

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  On the 126 mph run of LNER Class A4 4468 Mallard, a long British focus on speed reached its peak, and the record has never been broken. Claims that the Pennsylvania Railroad class S1 reached speeds above 150 mph have circulated for years, but they were never officially proven.

  American railways chose size instead. Larger loading gauges allowed enormous machines such as the Union Pacific Big Boy, which weighs 540 long tons and has a tractive effort of 135,375 lbf.

  From the early 1900s, electric and diesel locomotives began to replace steam. Railways started converting fully to electric and diesel power in the late 1930s. By the 1980s the majority of steam locomotives had left regular service, though several keep running on tourist and heritage lines.

  Preserving them on the mainline now takes more than coal and water. Because so much of the original operating infrastructure and staffing has been lost, preserved steam locomotives running on the mainline often travel with a support crew. In 2021 the restored Union Pacific 4014 was among the first mainline locomotives fitted with positive train control, bringing a 21st-century safety system to a machine from the age of steam.

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