The earliest railways employed horses to draw carts along railed tracks.
As the development of steam engines progressed through the 1700s, various attempts were made to apply them to road and railway use. The first attempts were made in Great Britain; the earliest steam rail locomotive was built in 1804 by Richard Trevithick and Andrew Vivian. It ran with mixed success on the narrow gauge "Penydarren tramroad" at Merthyr Tydfil in Wales.. Then followed the successful twin cylinder locomotive by Christopher Blackett's team built at Wylam in 1811, closely followed by Matthew Murrays' rack locomotive for the edge railed Middleton Railway in 1812 . These early efforts culminated in 1829 with the Rainhill Trials and the opening of the Liverpool and Manchester Railway a year later making exclusive use of steam power for both passenger and freight trains.
The United States started developing steam locomotives in 1829 with the Baltimore and Ohio Railroad's Tom Thumb. This was the first locomotive to run in America, although it was intended as a demonstration of the potential of steam traction, rather than as a revenue-earning locomotive. The first successful steam railway in the US was the South Carolina Railroad whose inaugural train ran in December 1830 hauled by the Best Friend of Charleston. Many of the earliest locomotives for American railroads were imported from England, including the Stourbridge Lion and the John Bull, but a domestic locomotive manufacturing industry was quickly established, with locomotives like the DeWitt Clinton being built in the 1830s.
The cylinders are double acting, with steam admitted to each side of the piston in turn. In a two-cylinder locomotive, one cylinder is located on each side of the locomotive. The cranks are set 90° out of phase with each other. During a full rotation of the driving wheel, steam provides four power strokes per revolution; that is to say each cylinder receives two injections of steam. The first stroke is to the front of the piston and the second stroke to the rear of the piston; hence two working strokes. Consequently two deliveries of steam onto each piston face in two cylinders generates a full revolution of the driving wheel. Each piston is connected to the driving axle on each side by a connecting rod, the driving wheels are connected together by coupling rods to transmit power from the main driver to the other wheels. At the two "dead centres", when the connecting rod is on the same axis as the crankpin on the driving wheel, it will be noted that no turning force can be applied. If the locomotive were to come to rest in this position it would be impossible for it to move off again, so the cylinders and crankpins are arranged such that the dead centres occur out of phase with each other. This precaution is unnecessary on most other reciprocating engines (such as an internal combustion engine) which are never expected to start from rest under their own power, and employ a flywheel to overcome the dead centres.
Each piston transmits power directly through a connecting rod (US: main rod) and a crankpin (US: wristpin) on the driving wheel (US main driver) or to a crank on a driving axle. The movement of the valves in the steam chest is controlled through a set of rods and linkages called the valve gear, actuated from the driving axle or else from the crankpin; the valve gear includes devices that combine the roles of reversing the engine, adjusting valve travel and the timing of the admission and exhaust events. The cut-off point determines the moment when the valve blocks a steam port, "cutting off" admission steam and thus determining the proportion of the stroke, during which steam is admitted into the cylinder; for example a 50% cut-off admits steam for half the stroke of the piston. The remainder of the stroke is driven by the expansive force of the steam. Careful use of cut-off provides economical use of steam and, in turn, reduces fuel and water consumption. The reversing lever (US: Johnson bar), or screw-reverser, (if so equipped) which controls the cut-off therefore performs a similar function to a gearshift in an automobile- 100% cut-off , providing maximum tractive effort at the expense of efficency, is used to pull away from a standing start, whilst a cut-off as low as 10% is used when cruising, providing reduced tractive effort but with lower fuel/water consumption.
Exhaust steam is directed upwards to the atmosphere through the chimney, by way of a nozzle called a blastpipe that gives rise to the familiar "chuffing" sound of the steam locomotive. The blastpipe is placed at a strategic point inside the smokebox that is at the same time traversed by the combustion gases drawn through the boiler and grate by the action of the blast. The combining of the two streams is crucial to the efficiency of any steam locomotive and the internal profiles of the chimney, (or more strictly speaking, the ejector) require careful design and adjustment. This has been the object of intensive studies by a number engineers (and almost totally ignored by others with sometimes catastrophic effect). The fact that the draught depends on the exhaust pressure means that power delivery and power generation are automatically self-adjusting and among other issues, a balance has to be struck between obtaining sufficient draught for combustion whilst giving the gases and particles sufficient time to be consumed. In the past, fierce draught could lift the fire off the grate, or cause the ejecting of unburnt particles leading to the dirt and pollution for which steam locomotives had an unenviable reputation in the past. Moreover, the pumping action of the exhaust has the counter effect of exerting back pressure on the side of the piston receiving steam, thus somewhat reducing cylinder power. Designing the exhaust ejector has become a specific science in which Chapelon, Giesl and Porta were successive masters, and was largely responsible for spectacular improvements in thermal efficiency but drastic reduction in maintenance time and pollution. A similar system was used by some early gasoline/kerosene tractor manufacturers (Advance-Rumely/Hart-Parr) – the exhaust gas volume vented through a cooling tower meant that more exhaust drew more air past the radiator.
Locomotives with total adhesion, i.e. where all the wheels are coupled together, generally lack stability at speed. This makes desirable the inclusion of unpowered carrying wheels mounted on two-wheeled trucks or 4-wheeled bogies centred by springs that help to guide the locomotive through curves. These usually take the weight of the cylinders in front or of the firebox at the rear end when the width of this exceeds that of the mainframes. For multiple coupled wheels on a rigid chassis a variety of systems for controlled side-play exist.
The fuel source used depends on what is economically available locally to the railway. In the UK and parts of Europe, a plentiful supply of coal made this the obvious choice from the earliest days of the steam engine. German, Russian, Australian and British railways experimented using coal dust to fire locomotives. Up to around 1850 in the U.S.A the vast majority of locomotives burnt wood until most of the Eastern forests were cleared; from that time on coal burning became more widespread and wood burners were restricted to rural and logging districts. In Europe, this lasted well into the 20th century. Bagasse, a waste by-product of the refining process, was burned in sugar cane farming operations. In the USA, the ready availability of oil made this a popular steam locomotive fuel; the Southern Pacific, for example, went directly from wood to oil. equipment. In Victoria, Australia after World War II, many steam locomotives were converted to heavy oil firing.
A number of tourist lines and heritage locomotives in Switzerland, Argentina and Australia have been using light diesel-type oil..
Water was supplied at stopping places and locomotive depots from a dedicated water tower connected to water cranes or gantries. In the UK, the USA and France, water troughs (US track pans) were provided on some main lines to allow locomotives to replenish their water supply without stopping. This was achieved by using a 'water scoop' fitted under the tender or the rear water tank in the case of a large tank engine; the fireman remotely lowered the scoop into the trough, the speed of the engine forced the water up into the tank, and the scoop was raised again once it was full.
Water is an essential element in the operation of a steam locomotive; because as Swengel argued:
it has the highest specific heat of any common substance; that is more thermal energy is stored by heating water to a given temperature than would be stored by heating an equal mass of steel or copper to the same temperature. In addition, the property of vapourising (forming steam) stores additional energy without increasing the temperature...water is a very satisfactory medium for converting thermal energy of fuel into mechanical energy
Swengel went on to note that "at low temperature and relatively low boiler outputs" good water and regular boiler washout was an acceptable practise, even though such maintenance was high. As steam pressures increased, however, a problem of "foaming" or "priming" developed in the boiler, wherein dissolved solids in the water formed "tough-skinned bubbles" inside the boiler, which in turn were carried into the steam pipes and could blow off the cylinder heads. To overcome the problem, hot mineral concentrated water was deliberately wasted (blowing down) from the boiler from time to time. Higher steam pressures required more blowing down of water out of the boiler. Oxygen generated by boiling water attacks the boiler and with increased steam pressures the rate of rust (iron oxide) generated inside the boiler increases. One way to help overcome the problem was water treatment. Swengel suggested that the problems around water, contributed to the interest in electrification of railways.
In the 1970s L.D. Porta developed a sophisticated heavy duty chemical water treatment that not only keeps the inside of the boiler clean and prevents corrosion, but modifies the foam in such a way as to form a compact "blanket" on the water surface that filters the steam as it is produced, keeping it pure and preventing carry-over into the cylinders of water and suspended abrasive matter.
The lagging is protected by a close-fitted sheet-metal casing known as boiler clothing or cleading.
Effective lagging is particularly important for fireless locomotives; however in recent times under the influence of L.D. Porta, "exaggerated" insulation has been practised for all types of locomotive on all surfaces liable to dissipate heat, such as cylinder ends and facings between the cylinders and the mainframes. This considerably reduces engine warmup time with marked increase in overall efficiency.
The earliest locomotives did not show the pressure of steam in the boiler, but it was possible to estimate this by the position of the safety valve arm which often extended onto the firebox back plate; gradations marked on the spring column gave a rough indication of the actual pressure. The promoters of the Rainhill trials urged that each contender have a proper mechanism for reading the boiler pressure and Stephenson devised a nine-foot vertical tube of mercury with a sight-glass at the top, mounted alongside the chimney, for the Rocket. The Bourdon tube gauge, in which the pressure straightens an oval-section, coiled tube of brass or bronze connected to a pointer, was introduced in 1849 and quickly gained acceptance. This is the device used today. Some locomotives have an additional pressure gauge in the steam chest. This helps the driver avoid wheel-slip at startup, by warning if the regulator opening is too great.
Locomotives of the British Railways standard classes fitted with self-cleaning smokeboxes were identified by a small cast oval plate marked "S.C.", fitted at the bottom of the smokebox door. These engines required different disposal procedures and the 'S.C.' plate highlighted this need to depot staff.
The use of live steam and exhaust steam injectors also assists in the pre-heating of boiler feed water to a small degree, though there is no efficiency advantage to live steam injectors. Such pre-heating also reduces the thermal shock that a boiler might experience when cold water is introduced directly. This is further helped by the top feed where water is introduced to the highest part of the boiler and made to trickle over a series of trays. G.J. Churchward fitted this arrangement to the high end of his domeless coned boilers Other British lines such as the LBSCR fitted a few locomotives with the top feed inside a separate dome forward of the main one.
Steam locomotives consume vast quantities of water, and supplying this was a constant logistical problem. In some desert areas, condensing engines were devised. These engines had huge radiators in their tenders and instead of exhausting steam out of the funnel it was captured and passed back to the tender and condensed. The cylinder lubricating oil was removed from the exhausted steam to avoid a phenomenon known as priming, a condition caused by foaming in the boiler which would allow water to be carried into the cylinders causing damage because of its incompressibility. The most notable engines employing condensers (Class 25C) worked across the Karoo desert of South Africa, from the 1950 until the 1980s.
Some British and American locomotives were equipped with scoops which collected water from "water troughs" (US: "track pans") while in motion, thus avoiding stops for water. In the US, small communities often did not have refilling facilities. During the early days of railroading, the crew simply stopped next to a stream and filled the tender using leather buckets. This was known as “jerking water” and led to the term "jerkwater towns" (meaning a small town, a term which today is considered derisive). In Australia and South Africa, locomotives in drier regions operated with large oversized tenders and some even had an additional water wagon, sometimes called a "canteen" or in Australia (particularly in New South Wales) a "water gin".
Steam locomotives working on underground railways (such as London's Metropolitan Railway) were fitted with condensing apparatus for a different, but obvious, reason. These were still being used between King's Cross and Moorgate into the early 1960s.
An alternative to the air brake is the vacuum brake. Where vacuum brakes are used, a steam-operated ejector is mounted on the engine instead of the air pump. A secondary ejector or crosshead vacuum pump is used to maintain the vacuum in the system. Vacuum systems existed on British, Indian and South African rail networks.
Steam locomotives are nearly always fitted with sandboxes from which sand can be delivered to the rails to improve traction and braking in wet or icy weather. On American locomotives the sandboxes, or sand domes, are usually mounted on top of the boiler. In Britain, the limited loading gauge precludes this, so the sandboxes are mounted just above, or just below, the running plate.
As speeds and distances increased, mechanisms were developed that injected thick mineral oil into the steam supply. The first, a displacement lubricator, mounted in the cab, uses a controlled stream of steam condensing into a sealed container of oil. Water from the condensed steam displaces the oil into pipes. The apparatus is usually fitted with sight-glasses to confirm the rate of supply. A later method uses a mechanical pump worked from one of the crossheads. In both cases, the supply of oil is proportional to the speed of the locomotive.
Lubricating the frame components (axle bearings, horn blocks and bogie pivots) depends on capillary action: trimmings of worsted yarn are trailed from oil reservoirs into pipes leading to the respective component. The rate of oil supplied is controlled by the size of the bundle of yarn and not the speed of the locomotive, so it is necessary to remove the trimmings (which are mounted on wire) when stationary. However, at regular stops (such as a terminating station platform) oil finding its way onto the track can still be a problem.
Crank pin and crosshead bearings carry small cup-shaped reservoirs for oil. These have feed pipes to the bearing surface that start above the normal fill level, or are kept closed by a loose-fitting pin, so that only when the locomotive is in motion does oil enter. In United Kingdom practice the cups are closed with simple corks, but these have a piece of porous cane pushed through them to admit air. It is customary for a small capsule of pungent oil (aniseed or garlic) to be incorporated in the bearing metal to warn if the lubrication fails and excess heating or wear occurs.
In American practice all of the forces between the locomotive and cars are handled through the coupler and its associated draft gear, which allows some limited slack movement. Small dimples called "poling pockets" at the front and rear corners of the locomotive allowed cars to be pushed on an adjacent track using a pole braced between the locomotive and the cars.
Britain used low intensity oil lamps and were not intended to allow the driver to see the way ahead (locomotive drivers were expected to have sufficient route knowledge) but were used to indicate the class of a train by their position on the front of the locomotive. Four lamp irons were provided: one below the chimney and three evenly spaced across the top of the buffer beam. The exception to this was the Southern Railway and its constituents, who added two lamp irons one each side of the smokebox, and the arrangement of lamps (or in daylight white circular plates) told railway staff the origin and destination of the train.
In some countries heritage steam operation continues on the national network. Some railway authorities have mandated powerful headlights on at all times, including during daylight. This was to further inform the public or track workers of any active trains.
Early bells and whistles were sounded through pull-string cords and levers. As the steam era approached the 1950s, automatic air-operated bells were made use of on locomotives such as the Challenger (steam locomotive) and Big Boy.
Some locomotives received extra cylinders and experiments combined two locomotives in one (e.g. the Mallet and Garratt locomotives). Some locomotives carried their cylinders vertically alongside the boiler and drove the wheels through a system of shafts and gears (e.g. the Shay locomotive; see "geared steam locomotive").
From about 1930, most new British express passenger locomotives were 4-6-0 or 4-6-2 types with three or four cylinders. Examples include:
In the United States on the Southern Pacific Railroad a series of cab forward locomotives had the cab and the firebox at the front of the locomotive and the tender behind the smokebox, so that the engine appeared to run backwards. This was only possible by using oil-firing. Southern Pacific selected this design to provide smoke-free breathing for the locomotive's engineer as they went through the SP's numerous mountain tunnels and snow sheds. Another variation was the Camelback locomotive with the cab half-way along the boiler.
Steam turbines were one of the experiments in improving the operation and efficiency of steam locomotives. Experiments with steam turbines using direct-drive and electrical transmissions, in different countries, proved mostly unsuccessful. The LMS also built Turbomotive, a largely successful attempt to prove the efficiency of steam turbines. Had it not been for the outbreak of WW2, more may have been built. The Turbomotive ran from 1935-49, when it was rebuilt into a conventional locomotive because replacement of many parts was required, an uneconomical proposition for a 'one-off' locomotive. In the United States the Union Pacific, Chesapeake and Ohio, and Norfolk & Western railways all built turbine-electric locomotives. The Pennsylvania Railroad (PRR) also built turbine locos but with a direct-drive gearbox. However, all designs failed due to dust, vibration, design flaws, or inefficiency below speed. The last one in service was the N&W's being retired in January 1958.
Numerous technological advances improved the steam engine. Early locomotives used simple valve gear that gave full power in either forward or reverse. Soon Stephenson valve gear allowed the driver to control cut-off; this was largely superseded by Walschaerts valve gear and similar patterns. Early locomotive designs using slide valves and outside admission were easy to construct, but inefficient and prone to wear. Eventually, slide valves were superseded by inside admission piston valves, though there were attempts to apply poppet valves (common by then on stationary engines) in the 20th century. Stephenson valve gear was generally placed within the frame and was difficult to access for maintenance; later patterns applied outside the frame, were readily visible and maintained.
From 1876, compound locomotives came on the scene, which used the engine's steam twice. There were many compound locomotives especially where long periods of continuous efforts were needed. Compounding was an essential ingredient of the quantum leap in power achieved by André Chapelon's rebuilds from 1929. A common application was to articulated locomotive, the most common being that of Anatole Mallet in which the high pressure stage was attached directly to the boiler frame; in front of this was pivoted a low pressure engine on its own frame, taking the exhaust from the rear engine.
Articulation itself proved very popular, and there were numerous variations, both compound and simple. Duplex locomotives with two engines in one rigid frame were also tried, but were not notably successful. For example, the 4-4-4-4 Pennsylvania Railroad's T1 class, designed for very fast running, suffered incurable slippage problems throughout their careers.
With the notable exception of the USRA standard locomotives, set during World War 1, in the United States, steam locomotive manufacture was always customised. Railroads ordered locomotives tailored to their specific requirements, though basic similarities were always present. Railroads developed specific characteristics; for example, the Pennsylvania Railroad had a preference for the Belpaire firebox, while the Delaware and Hudson Railroad was famous for its elaborately flanged smokestacks. In the United States, specialised manufacturers constructed locomotives for all rail companies, although all railroads had shops capable of heavy repairs and some railroads (for example the Norfolk and Western Railway) constructed locomotives in their own shops. Companies Manufacturing locomotives in the US included Baldwin Locomotive Works, American Locomotive Works (ALCO), Lima Locomotive Works, and others. It was not uncommon for an entire group of locomotives to be sold from one railroad to another.
Steam locomotives required regular service and overhaul (often at government-regulated intervals). Many alterations occurred during overhauls. New appliances were added, unsatisfactory features removed, cylinders improved or replaced. Any part of the locomotive, including boilers were replaced or upgraded. On the Baltimore and Ohio Railroad two 2-10-2 locomotives were dismantled; the boilers were placed onto two new Class T 4-8-2 locomotives and the residue wheel machinery made a pair of Class U 0-10-0 switchers with new boilers. Union Pacific's fleet of 3 cylinder 4-10-2 engines were converted into two cylinder engines in 1942, because of high maintenance problems.
Before the 1923 Grouping, the picture in the UK was mixed. The larger railway companies built locomotives in their own workshops but the smaller ones and industrial concerns ordered them from outside builders. A large market for outside builders was abroad because of the home-build policy exercised by the main railway companies.
Between 1923 and 1947, the "Big Four" railway companies (the Great Western Railway, the London, Midland and Scottish Railway, the London and North Eastern Railway and the Southern Railway) all built most of their own locomotives. Generally speaking, they only bought locomotives from outside builders when their own works were fully occupied (or as a result of government-mandated standardisation during wartime).
From 1948, British Railways allowed the former "Big Four" companies (now designated "Regions") to continue to build their own designs, but also created a range of "standard" locomotives which supposedly combined the best features from each region. Although a policy of "dieselisation" was adopted in 1955, BR continued to build new steam locomotives until 1960 (the last being named Evening Star).
Some independent manufacturers produced steam locomotives for a few more years, the last British-built industrial steam locomotive being constructed by Hunslet in 1971. Since then, a few specialised manufacturers have continued to produce small locomotives for narrow gauge and miniature railways, but as the prime market for these is the tourist and heritage railway sector, the demand for such locomotives is limited.
In Australia, Clyde Engineering of Sydney and also the Eveleigh Workshops built steam locomotives for the New South Wales Government Railways. These include the C38 class 4-6-2, the first five were build at Clyde with streamlining, the other 25 locomotives were built at Eveleigh (13) in Sydney, and Cardiff Workshops (12) near Newcastle. In Queensland, steam locomotives were locally constructed by Walkers. Similarly the South Australian state government railways also manufactured steam locomotives locally at Islington in Adelaide. The Victorian Railways constructed most of their locomotives at their Newport Workshops and Bendigo while in the early days locomotives were built in Ballarat. Locomotives constructed at the Newport shops ranged from the nA class 2-6-2T built for the narrow gauge, up to the H class 4-8-4, the largest conventional locomotive ever to operate in Australia, which weighed 260 tons. However, the title of largest locomotive in Australia goes to the 263 ton NSWGR AD60 class 4-8-4+4-8-4 Garratt (Oberg:1975), which were built by Beyer-Peacock in the United Kingdom.
The Whyte notation, used in most English speaking and Commonwealth countries, represents each set of wheels with a number. Different arrangements were given names which usually reflect the first usage of the arrangement; for instance the "Santa Fe" type (2-10-2) is so called because the first examples were built for the Atchison, Topeka and Santa Fe Railroad. These names were informally given and varied according to region and even politics.
The UIC classification is used mostly in European countries apart from the United Kingdom. It designates consecutive pairs of wheels (informally "axles") with a number for non-driving wheels and a capital letter for driving wheels (A=1, B=2 etc). So a Whyte 4-6-2 designation would be an equivalent to a 2-C-1 UIC designation.
On many railroads, locomotives were organised into classes. These broadly represented locomotives which could be substituted for each other in service, but most commonly a class represented a single design. As a rule classes were assigned some sort of code, generally based on the wheel arrangement. Classes also commonly acquired nicknames representing notable (and sometimes uncomplimentary) features of the locomotives.
In the steam locomotive era, two measures of locomotive performance were generally applied. At first, locomotives were rated by tractive effort This can be roughly calculated by multiplying the total piston area by 85% of the boiler pressure (a rule of thumb reflecting the slightly lower pressure in the steam chest above the cylinder) and dividing by the ratio of the driver diameter over the piston stroke. However, the precise formula is:
Tractive Effort is defined as the average force developed during one revolution of the driving wheels at the rail head. This is expressed as:
where d is bore of cylinder (diameter) in inches, s is cylinder stroke, in inches, P is boiler pressure in pound per square inch, D is driving wheel diameter in inches, c is the effective cut-off.
It is critical to appreciate the use of the term 'average', as not all effort is constant during the one revolution of the drivers for at some points of the cycle only one piston is exerting turning moment and at other points both pistons are working. Not all boilers deliver full power at starting and also the tractive effort decreases as the rotating speed increases.
Tractive effort is a measure of the heaviest load a locomotive can start or haul at very low speed over the ruling grade in a given territory.
However, as the pressure grew to run faster freight and heavier passenger trains, tractive effort was seen to be an inadequate measure of performance because it did not take into account speed.
Therefore in the 20th century, locomotives began to be rated by power output. A variety of calculations and formulas were applied, but in general railroads used dynamometer cars to measure tractive force at speed in actual road testing. This measure was termed drawbar horsepower in the United States and remained the standard measure of performance to the end of mainline usage.
British railway companies have been reluctant to disclose figures for drawbar horsepower and have usually relied on continuous tractive effort instead.
Whyte classification is connected to locomotive performance, but through a somewhat circuitous path. Given adequate proportions of the rest of the locomotive, power output is determined by the size of the fire, and for a bituminous coal-fuelled locomotive, this is determined by the grate area. Modern non-compound locomotives are typically able to produce about 40 drawbar horsepower per square foot of grate. Tractive force, as noted earlier, is largely determined by the boiler pressure, the cylinder proportions, and the size of the driving wheels. However, it is also limited by the weight on the driving wheels (termed adhesive weight), which needs to be at least four times the tractive effort.
The weight of the locomotive is roughly proportional to the power output; the number of axles required is determined by this weight divided by the axleload limit for the trackage where the locomotive is to be used. The number of driving wheels is derived from the adhesive weight in the same manner, leaving the remaining axles to be accounted for by the leading and trailing bogies. Passenger locomotives conventionally had two-axle leading bogies for better guidance at speed; on the other hand, the vast increase in the size of the grate and firebox in the 20th century meant that a trailing bogie was called upon to provide support. On the European continent, some use was made of several variants of the Bissel bogie in which the swivelling movement of a single axle truck controls the lateral displacement of the front driving axle (and in one case the second axle too). This was mostly applied to 8-coupled express and mixed traffic locomotives and considerably improved their ability to negotiate curves whilst restricting overall locomotive wheelbase and maximising adhesion weight.
As a rule, "shunting engines" (US "switching engines") omitted leading and trailing bogies, both to maximise tractive effort available and to reduce wheelbase. Speed was unimportant; making the smallest engine (and therefore smallest fuel consumption) for the tractive effort paramount. Driving wheels were small and usually supported the firebox as well as the main section of the boiler. Banking engines (US "helper engines") tended to follow the principles of shunting engines, except that the wheelbase limitation did not apply, so banking engines tended to have more driving wheels. In the US, this process eventually resulted in the Mallet type with its many driven wheels, and these tended to acquire leading and then trailing bogies as guidance of the engine became more of an issue.
As locomotive types began to diverge in the late 1800s, freight engine designs at first emphasised tractive effort, whereas those for passenger engines emphasised speed. Over time, freight locomotive size increased, and the overall number of axles increased accordingly; the leading bogie was usually a single axle, but a trailing truck was added to larger locomotives to support a larger firebox that could no longer fit between or above the driving wheels. Passenger locomotives had leading bogies with two axles, fewer driving axles, and very large driving wheels in order to limit the speed at which the reciprocating parts had to move.
In the 1920s the focus in the United States turned to horsepower, epitomised by the "super power" concept promoted by the Lima Locomotive Works, although tractive effort was still the prime consideration after World War One to the end of steam. Freight trains were to run faster; passenger locomotives needed to pull heavier loads at speed. In essence, the size of grate and firebox increased without changes to the remainder of the locomotive, requiring the addition of a second axle to the trailing truck. Freight 2-8-2s became 2-8-4s while 2-10-2s became 2-10-4s. Similarly, passenger 4-6-2s became 4-6-4s. In the United States this led to a convergence on the dual-purpose 4-8-4 and the 4-6-6-4 articulated configuration, which was used for both freight and passenger service. Mallet locomotives went through a similar transformation and evolved from bank engines into huge mainline locomotives with gargantuan fireboxes; their driving wheels being increased in size in order to allow faster running.
Mainline diesel-electric locomotives first appeared on the Baltimore and Ohio Railroad, in 1935 as locomotive No. 50. The diesel reduced maintenance costs dramatically, while increasing locomotive availability. On the Chicago, Rock Island and Pacific Railroad the new units delivered over a year, compared with about 120,000–150,000 for a mainline steam locomotive. World War II delayed dieselisation in the U.S.A, but the pace picked up in the 1950s. Among large railroads, the Alabama, Tennessee and Northern Railroad; and the Western Pacific Railroad were among the first to completely retire steam power. By 1960, the last American Class I holdout, the Norfolk and Western Railway, discontinued steam operations. Some U.S. shortlines continued steam operations into the 1960s, and the Northwestern Steel and Wire mill in Sterling, IL, continued to operate steam locomotives until December 1980.
Trials of diesel locomotives and railcars began in the United Kingdom in the 1930s but made only limited progress. One problem was that British diesel locomotives were often seriously under-powered, compared with the steam locomotives against which they were competing.
After 1945, problems associated with post-war reconstruction and the availability of cheap domestic-produced coal kept steam in widespread use throughout the two following decades. However the ready availability of cheap oil led to new dieselisation programmes from 1955 and these began to take full effect from around 1962. Towards the end of the steam era, which came about in 1968, steam motive power was allowed to fall into a dire state of repair; this along with the absence of attention given to the attendant staff working conditions could only accelerate the decline to such a degree that British Railways estimated that its steam locomotives accounted for around four times more in running costs than diesels. The use of steam locomotives in British industry continued on an ever-reducing scale into the late 1980s, but the poor availability of replacement parts, coupled with the decline of the coal mining industry, led to the disappearance of steam power for commercial uses.
In other countries, the conversion from steam was slower. By March 1973 in Australia, steam had vanished in all states. Diesel locomotives were more efficient and the demand for manual labour for service and repairs was less than steam. Cheap oil had cost advantages over coal.
In the USSR, the last steam locomotive (П36, serial number 251) was built in 1956; now in the Museum of Railway Machinery at former Warsaw Rail Terminal, Saint Petersburg, Russia. In the European part of the USSR, almost all steam locomotives were replaced by diesel and electrical ones in 1960s; in Siberia with its cheap coal, steam locomotives were in active use till mid-1970s. However, some photographs exist of Russian steam locomotives at work into the 1980s, and many accurate historical records state that Russian Decapods, L-class 2-10-0s, and LV-class 2-10-2s were retired between 1980-1985, implying that the best of Russian steam, such as the P36 class, remained on the active rosters into the 1990s. Until 1994, Russia had at least 1,000 steam locomotives stored in operable condition in case of "national emergencies" - as a result, more than 200 steam locomotives are still in working condition.
In Poland, on non-electrified tracks steam locomotives were superseded almost entirely by diesels by the early '90s. A few steam locomotives, however, operate still from Wolsztyn. Although they are maintained operational rather as a means of preserving railway heritage and as a tourist attraction, they do haul regular scheduled trains (mostly to Poznań). Apart from that, numerous railway museums and heritage railways (mostly narrow gauge) own steam locomotives in working condition.
In India steam locomotives were in use until 1989; they were replaced by a combination of diesel and electric locomotives. Steam locomotives from India were then sold to an undisclosed African country. A steam locomotive celebration run was organised between Thane and Mumbai to commemorate the 150th year of railways in India.
In South Africa an oil embargo combined with an abundance of cheap local coal and a cheap labour force, ensured steam locomotives survived into the 1990s. Locomotive engineer L. D. Porta's designs appeared on a Class 19D engine in 1979, then a former Class 25 4-8-4 engine, became a Class 26, termed the "Red Devil" No. 3450, which demonstrated an improved overall performance with decreased coal and water consumption. The single class 26 locomotive operated until the end of steam. Another class 25NC locomotive, No. 3454, nicknamed the "Blue Devil" because of its colour scheme, received modifications including a most obvious set of double side-by-side exhaust stacks. In southern Natal, two former South African Railway gauge NGG16 Garratts operating on the privatised Port Shepstone & Alfred County Railway (ACR) received some L. D. Porta modifications in 1990 becoming a new NGG16A class.
China continued to build mainline steam locomotives until late in the century, even building a few examples for American tourist operations. Since China was the last main-line user of steam locomotives, ending officially at the beginning of 2006, it is plausible that many still exist in industrial operations or in more remote parts of China. Many coal mines and smaller cities, such as Pingdingshan and Hegang, maintain an active roster of JS, SY, or QJ steam locomotives bought secondhand from China Rail. The last steam locomotives built in China were of the SY 2-8-2 class, built until 1999. The last steam locomotive built in China was SY 1772, finished in 1999. As of 2007, at least four Chinese steam locomotives exist in the United States - 2 QJ's bought by RDC, a JS bought by the Boone Scenic Railway, and an SY bought by the NYSW for tourist operations, but re-painted and modified to represent a 1920s era US locomotive.
Pakistan is currently the only country where there is still a regular steam locomotive service; a line operates in the Northwest Frontier Province and Sindh; It has been preserved as a "nostalgia" service for tourism in exotic locales, indeed it is specifically advertised as being for "steam buffs
Several heritage railways in the United Kingdom have built new steam locomotives in the 1990s and early 2000s. These include the narrow gauge Ffestiniog and Corris railways in Wales. The Hunslet Engine Company was revived in 2005 and is now building steam locomotives on a commercial basis. A standard gauge LNER Peppercorn Pacific Tornado was completed at Hopetown Works, Darlington, England and made its first run on 1 August 2008. It will enter mainline service later in 2008, with demonstration trips in France and Germany planned.
Steam locomotives have left such a legacy behind, that they have been the main topic for numerous collectors and bullion coins. On of the most recent ones is the famous 20 euro Biedermeier Period coin, minted in June 11 2003. The obverse of the coin shows a premature steam locomotive (the AJAX) on Austria's first railway line, the Kaiser Ferdinand's Nordbahn. The AJAX can still be seen today in the Austrian railway museum.