Every steam engine that has ever worked the iron road has had wheels, at least four if it was in any way functional. The arrangement and size of these wheels determines the role that a locomotive would play within the company, from heavy freight lugger to flying express train. So if you've always wondered what advantage a 0-6-0 has over a 2-2-2 and what the difference between an Atlantic and a Pacific is, then this is the entry for you.
While steam engines were used all around the world, this entry will mainly use British locomotives as examples, with some references to American engines.
Which Wheel Is Which - Notation
Most normal, non-articulated1 locomotives have up to three sets of wheels. They are:
The leading truck: Small wheels at the front of the train. They do take some of the weight of the locomotive, but their general purpose is to guide the train around curves and over points. They both pivot on the frame and can move slightly from side to side to better navigate any difficulties the track may throw up. While slower trains may not have them, they were considered almost vital for fast passenger locomotives, so that speed could be maintained safely.
The driving wheels: The large, often coupled, wheels through which the power is transferred. The size of these wheels often dictates use. Large wheels are better for high speed, as they don't have to rotate as much. Smaller wheels can give better acceleration, but the main reason for their use is that a lot of small wheels give much better traction than one large wheel. At the dawn of the railway age, a passenger locomotive may have got by with only two driving wheels, but as loads got heavier, they needed more and more. Another disadvantage of large driving wheels is that if they're too tall, they restrict the space that can be used for the boiler.
The trailing truck: The small wheels underneath the cab. These wheels provide support for the cab and allow the firebox to be moved back from its original position between the driving wheels, allowing it to be much larger. Without a trailing truck, the firebox either had to go between the driving wheels, so would be narrow but deep, or hang unsupported behind the driving wheels, so shallow but wide. Tank engines, that is, engines that carry their own coal and water on board, rather than in a tender, may often have a trailing truck to support the extra weight of the coal bunker on the rear of the locomotive.
Not all railway lines were built to the same standard, and one limiting factor in which locomotives could use each line was the axle weight, that is the weight coming down through the axle onto the rail. Having more axles means that the weight is spread, so axle weight is lower. Conversely, heavy freight locomotives would need all their mass over driven axles, so may not have had a leading or trailing truck.
So, how do we describe these arrangements? While some prefer to count the number of axles in each set, the main method, known as the Whyte system, is to count the number of wheels. For instance, a 4-6-2 has four leading wheels, six driven wheels and a pair of trailing wheels. This can also be represented 'graphically' as ooOOOo.
Tank engines, locomotives that hold their water in tanks next to the boiler and their coal in bunkers behind the cab, are identified by the suffix T, e.g. a 2-6-2T. Saddle tanks, where the water tanks wrap over the boiler are STs (0-4-0ST) and pannier tanks, with box-shaped water tanks on either side of the boiler, are known as PTs.
In some countries the UIC system is used, in which unpowered axles are represented by numbers and powered axles by letters. Here the 4-6-2 arrangement would be a 2C1. This system is used in the UK to describe diesel and electric locomotives. While the UIC system has many add-on notations, the only one used in the UK is the o symbol to show that all the powered axles have their own motors. Only a few arrangements are seen in the UK, the main ones being Bo-Bo, Co-Co and A1A-A1A.
To help classify locomotives, the London Midland and Scottish Railway (LMS) introduced a system of power classification that was later adapted by British Rail. Locomotives were given two ratings, from 0 to 82, according to how much tractive effort (force) they managed at 50mph and 25mph. The first was the passenger rating, the second was the freight rating.
A locomotive classed P4 would produce between 6,720 and 7,839lbs at 50mph, which equated to around 896 and 1,045 horsepower. A P8 would be anything over 11,200lbs (1,493hp). An F4 would output between 11,425 and 13,104lbs at 25mph (761-873hp). For example, a locomotive might be rated as 7P6F, a seven for passenger, six for freight. If the ratings were equal in both categories, it would be classed as a mixed traffic locomotive, able to perform equally as well with freight and passengers. This would be marked with an MT suffix.
Limitations in early technology meant that the best way of getting a train to go fast was by using one pair of very large wheels that rotated relatively slowly, thereby putting less strain on the lubrication of the axle and cylinder.
With wheels that could be in excess of 8ft, each rotation covered a great distance. The sheer size of the wheels meant that boilers could not stretch across the full width of the engine because of the wheels being in the way. This limited the potential of the designs. As railways became more popular, train loads got heavier and the age of the single was over. More traction was needed, and two sets of smaller wheels were much better than one large one for tough loads. On the later single models, only a quarter of the engine's weight is over the driving wheel, so they were liable to wheelslip3. Singles were never very well suited to freight use, where traction is more important than speed.
0-2-2 is probably the simplest arrangement, with two driving wheels and two following. This was the layout for Stephenson's Rocket. Stephenson followed the Rocket with the Planet type, these were 2-2-0 locomotives. The American pioneering locomotive Tom Thumb was also a 2-2-0.
The 2-2-2 or Patentee was the evolution of the 2-2-0. Some wheel arrangements are given specific names, and all 2-2-2s are named after Stephenson's Patentee. With its large single set of driving wheels it was a successful express design. Great Western designs on their broad-gauge track, such as the Ixion and Great Western, set speed records, with the latter managing over 74mph.
The 4-2-2 soon became the main express arrangement for British locomotives. These were ideal locomotives for their time. The four leading wheels did a better job of nudging a train into a curve than the single axle truck. Great Western's Iron Duke of 1847 had driving wheels of 8ft in diameter, and one of its class, Great Britain, recorded a speed of 78.2mph in 1848. One of the most elegant locomotives ever produced were the Stirling 8-foot singles of the Great Northern Railway. These, resplendent in their light green paint, were built from 1870 onwards and were used on the London-Edinburgh expresses, including the Flying Scotsman4. Their 8ft 1in wheels saw them pulling loads at up to 85mph. This high-speed running, however, proved to be their downfall. Trains got heavier and heavier and the limited grip of the single saw them replaced by coupled wheeled engines on the prestigious expresses. The last express singles to be built were the Johnson Spinners of the late 1890s, made for the Midland Railway, which took advantage of advances in sanding technology to pull heavier loads and reach speeds of 90mph and above.
Many of the original locomotives used pairs of coupled driving wheels. With the power spread between four wheels, they could get more traction. While, in the early days, four coupled engines were used for freight, this market quickly moved onto the six coupled engines, however four coupled trains carried passenger services well into the 20th Century.
0-4-0 was used by Locomotion. Many small tank engines were built as 0-4-0s, where they would be used either on tight, twisty tracks such as in docks or as low-cost shunters.
One advantage of the 'Patentee' frame was that you could remove the front truck and large driving wheels and replace them with four smaller, coupled, wheels to produce a freight locomotive like the Lion, a 0-4-2 locomotive that was the star of the 1953 Ealing Studios film The Titfield Thunderbolt.
2-4-0s like the London and North Western's 1874 'Precedent' class took over as the main express locomotives from the singles. These could still run with large (6ft 7in) driving wheels, but since they were coupled, they could pull much heavier loads.
The 4-4-0 was the arrangement that opened up a continent. This was the classic 'American' layout, countless numbers of these fast running locomotives with their giant smokestacks opened up the United States. By the 1870s, however, demand for bigger, heavier trains put pay to the layout. Over the pond, the 4-4-0s were the predominant fast running express locomotives around the turn of the century. Possibly the most famous implementation was the Great Western Railway's 'City' class. City of Truro became the first locomotive to pass 100mph in 1904.
The last, and largest, of this type built in Europe were the 'Schools' class, built for the Southern Railway. On lines where axle weight didn't matter, but where short wheelbases did, the 1930 design was the most powerful of its type ever made.
While 2-4-2s often proved unstable, the 4-4-2 'Atlantic' was widely used as an express passenger engine, especially in America where the streamlined Milwaukee Road Hiawatha engines were the first designed to run at 100mph day to day. Only a few classes of Atlantics ran in the UK.
Some local passenger services, especially in the UK, saw service with 0-4-4 locomotives, especially tank engines. These engines were able to run equally well in either direction. The Metropolitan Railway was one of the more famous users.
Six Coupled Engines
The 0-6-0 is probably the most familiar layout to most people. It originally started out as a heavy freight design with locomotives like Samson, but it was as tank engines, locomotives that didn't have a tender but had a small bunker to hold coal and water tanks around the boiler, that they are most famous. Beloved children's character Thomas the Tank Engine is a 0-6-0, as was Duck. Their six small coupled wheels meant that the fabulous little 24-ton Terrier tanks of the 1870s could produce immense feats of acceleration to stick to the rigid timetables of the London, Brighton and South Coast Railway's suburban services, which also included the gradients of Brunel's Thames Tunnel. The last British steam 0-6-0s were the Southern Railway's Q1s; these stark locomotives, built in 1942 look unlike everything else due to them being designed with the limited resources of wartime. 0-6-0s were the mainstay of many railways freight operations in tank and tender forms, especially on the smaller branch lines were larger locomotives could not get around the curves. The 0-6-0 is the only wheel arrangement still in regular use on mainline British railways. The small diesel shunters that are seen around depots are classified as 0-6-0s.
This design is not useful for high-speed running since it has no leading or trailing truck and therefore giving an unstable ride. With all the weight over the driven wheels, they could pull large loads. A lot of times they were employed in goods yards where speed was not an issue, they were ideal because they were the middle ground between four coupled tanks, which were normally too small to be powerful enough to move heavy trains, and the eight coupled tanks which often were too expensive and too long for tight curves which were common in marshalling yards and docks. 0-6-0s were also great for branch and suburban passenger services where lots of stops meant that acceleration was more important than top speed.
Among the countless designs, one of the most popular were the Great Western Pannier Tanks. These had box shaped water tanks on each side of the boiler, allowing access to the mechanisms under the engine. This was the type of engine that Thomas the Tank Engine's pal Duck was based on. Over 1,200 Pannier Tanks were made by the Great Western Railway.
The first 2-6-0 'Moguls' appeared in the USA in the 1850s and 100 years later 'Moguls' were still being produced by British Rail. These are primarily fast freight and mixed traffic engines, where most of the weight is over the powered wheels. When, in the 1950s, British Rail produced 12 standard classes of locomotives, three of these were 'Moguls', of classifications 2MT, 3MT and 4MT, while it is arguable if all of these were needed5, they proved handy engines, especially the smallest classes which had low enough axle loading to go almost anywhere in the network.
2-6-2 locomotives first appeared in New Zealand with the Central Pacific Railway in the 1880s. It was once they were used for trains across the vast prairies of the American Midwest with the Chicago, Burlington and Quincy Railroad in the 1900s that they became known as 'Prairies'. In the US, tender engine versions were popular, but in the UK, 2-6-2s were mostly tank engines.
Their major problem was that the layout was symmetrical, with the centre of gravity above the middle driving wheel, so that the motion of the connecting rods resulting in an uncomfortable rocking motion. This meant that unless they were engineered skilfully, they were not suitable for passenger use.
The London Midland and Scottish and Great Western Railways both had fleets of 'Prairie' tank engines which were useful for push-pull services on branch lines. Two classes of British Standard Prairie tanks were also made, classified as 2MT and 3MT, as the counterparts to the 'Mogul' tender engines. Sir Nigel Gresley designed a 2-6-2 tender engine for express and mixed traffic duties for the London and North Eastern. The V2s, including Green Arrow, first appeared in 1936 and were regarded as highly as the more famous Gresley Pacific locomotives, and could even deputise for them on fast expresses.
One notable inclusion to the pantheon of British 2-6-2s was the Paget engine designed for the Midland Railway by Cecil Paget. He had recently been promoted over his former boss Richard Deeley, the Locomotive Superintendent, and had proceeded to make away much of Deeley's responsibility's by taking direct control of the train crews in order to sort out the incredible complexity of the railway's timetable. The Paget engine had eight horizontal cylinders6, and was designed to remove the hammer effect that unbalanced cylinders have on the rails. What should have been a revolutionary engine failed on its first run and was quickly hushed up. It failed due to some of its parts seizing. The responsibility for building the locomotive had been Deeley's. Deeley was one of the country's leading experts on lubrication, and so a cynic may suggest that it was not an accident that this engine failed.
Hundreds of 2-6-4 tanks were used in the UK railways. These were some of the largest tank engines in operation, with the British Rail Class 4 tank engine tipping the scales at 86 tons.
The 4-6-0 first appeared in America in the 1840s, and pretty much replaced the ubiquitous 4-4-0s into the 1880s. Imaginatively, they were know as 'Ten Wheelers'. These were ideal fast freight and express passenger engines. In the UK three of the big four companies used these engines as their major express locomotives. Great Western Railway introduced the 1902 'Star' class, and then built on it with the 1923 'Castles' and 1937 'Kings'. Both of the later were, at one time or another, the most powerful locomotives in the country. Compared with the larger Pacific locomotives, 4-6-0s could not use as large a firebox, so could not generate as much steam at high speeds. On the other hand, they were able to put much more of their weight though their driving wheels, cancelling a lot of wheelslip. The Great Western Railway had access to Welsh Coal, which had a higher calorific value than other coal, so the smaller firebox limitations didn't affect them until they were forced to use normal coal after the Second World War.
Southern Railway had their 1919 'King Arthur'7 and 1926 'Lord Nelson' classes, the later which took GWR's power crown from the 'Castles' until the 'Kings' were introduced. LMS introduced the 1927 'Royal Scot' class for their premier expresses, which, went rebuilt, were probably the most powerful express 4-6-0s in the country. LMS's lesser expresses were handed by 'Jubilee' and 'Patriot' classes.
In 1928 Charles Collett, the chief designer of Great Western Railway, rebuilt a 'Saint' class locomotive with smaller wheels and other advances to produce the first 'Hall' class locomotive. Classified as 5MT, it was equally as good at passenger and freight haulage, and became the first true Mixed Traffic Engine. Olton Hall, one of the class, is the engine used in the Harry Potter films for the Hogwarts Express. Three hundred and thirty 'Halls' and 'Modified Halls' were built along with batches of 80 and 30 of the smaller 'Grange' and 'Manor' classes respectively and 30 larger 'County' class. Two other railways saw the benefits of these 'do anything' engines that had relatively few weight restrictions and based their 4-6-0 5MT designs on the GWR Halls, albeit slightly narrower to fit the loading restrictions of their railways. Edward Thompson took a break from trying to write Gresley out of railway history to produce his only competent design, the 'B1' for LNER, in 1942. LMS, with William Stanier on the drawing board, came out with the 1934 'LMS Class 5', known as the 'Black Five'. Eight hundred and forty two of these engines were built, one of the largest fleets of any class of locomotive. Additionally, Henry, in the Thomas the Tank Engine stories was rebuilt as a Black Five, this was in order to help the illustrators stick to one design throughout the books.
When the British Rail Standards plan was being drawn up, it was decided that even though around 1700 5MT 4-6-0s were running on the network, they needed another 172, which became the 'Standard Class 5', based on Stanier's LMS designs. To complement them, there were the lighter 4MT 'Class 4 4-6-0s'. Whether two 4MT classes were needed in the Standard plan, a 4-6-0 and a 2-6-0, is open to question, especially as the main idea of this plan was the cut the number of classes on the system.
When it comes to people talking about the golden age of steam, when the LNER and LMS were trying to top each other on their Scotland bound expresses, when the prewar 1930s marvelled at the glamour of streamlining and airsmoothed engines, the most evocative word in railways was 'Pacific'. This was the name for the 4-6-2, perhaps the ultimate form of passenger engine.
The first 'Pacifics' were shipped across the ocean of that name to New Zealand in 1901, built by the Baldwin Locomotive Works of Philadelphia, Pennsylvania for the New Zealand Railways Department. They needed a large firebox to deal with the low grade fuel that was mined in New Zealand. Seven thousand or so were produced for North American users, where it was the major force for passenger trains in the first half of the 20th century.
The major benefit of the 'Pacific' was that large firebox, which allowed it to generate a lot of steam. With twelve wheels it was a smooth ride. Its failing was that less than half its wheels were powered. With the huge power that a 'Pacific' could produce, this meant that wheelslip could occur at anytime, be it pulling away from stations or at high speed.
In the UK all the four major companies built 'Pacifics'. Great Western was the first to try. George Jackson Churchward built the 1908 Great Bear, a one-off designed to be the largest locomotive in the county, something that Great Western's marketing department readily pointed out to anybody who would listen. However, because its axle loading was so high, it was restricted to being running between London Paddington and Bristol. Churchward's successor, Collett, finally got the former company flagship rebuilt as a 4-6-0 'Castle' Class, Viscount Churchill in 1924. This was the only 'Pacific' that GWR built, their access to high quality coal meant that they could use 4-6-0s.
Under the pen of Oliver Bulleid, the Southern Railway produced the airsmoothed8 'Merchant Navy' class or 'Spamcans' in 1941. These innovative locomotives were supplemented by the 'Light Pacifics' of the 'West Country' and 'Battle of Britain' classes which were express engines that were able to take on both branch line trains and freight duties.
LMS had former GWR man William Stanier design their 'Pacifics', to race up the West Coast to Scotland. The 1933 'Princesses Royals' or 'Lizzies', tipped the scales at over 104 tonnes with a 54-ton tender. The 13-strong class didn't last long as the flagships of the company as the 'Princess Coronation' Class class arrived in 1937. Know as the Dutchesses, the first batch of these arrived with glorious steamlining, and broke the world speed record with a 114 mph run in 1937. During the war they lost their streamlining for ease of maintenance reasons and perhaps with it lost the last bit of glory of the steam age.
Sir Nigel Gresley was an accomplished designer first with the Great Northern Railway, then with the LNER. Head and shoulders above all his over achievements in the public conscious were his two classes of 'Pacifics', including the two most famous locomotives in the county. His giant 'A1's ran along the East Coast mainline from Kings Cross to Scotland. The most famous member of this class was The Flying Scotsman, possibly the most famous engine in the world. Another A1 was Gordon, in the Thomas the Tank Engine stories. After being trialled against the lighter, more powerful and more economical GWR Pendennis Castle, the 'A1s' were rebuilt to become the 'A3s'. While still and 'A1', Flying Scotsman became the first locomotive to officially break the 100mph barrier, in 1934. 'A3' No.2750 Papyrus hit 108mph in 1935.
The next step, both to improve speed and economy, and to satisfy the marketing men was a streamlined 'Pacific'. Unlike Stanier, Greasley built the streamlining onto his 'A4' engines. While Silver Link broke the speed record with a 113mph in 1936, it was Mallard that made the class immortal with a run of 126mph in 1938. They remained the flagships of the East Coast Line, until the Deltics became the first diesel with the mix of power and lightness that could replace them.
After Gresley, LNER carried on with 'Pacifics'. Thomson tried, wherever he could get his hands on a bit of Gresley gear when nobody was looking, to build a successful 4-6-2, but failed. His successor was Arthur Peppercorn. His 'A1s' and 'A2s' were both magnificent engines. Sadly none of his 48 1948 'A1s' were preserved, but in 2008, after a 13-year build, 60163 Tornado was built. Only one of his 'A2s' was preserved, and its fate featured heavily on Children's BBC, as 60532 was called Blue Peter.
When British Rail took over from the four companies, it was sell served for flagship express locomotives, but just behind that in the roster, it was lacking power. The Standard Plan created two classes of 'Pacific', the 'Class 7' 7P 'Britannias' and the 'Class 6' 6P 'Clans'. After an LMS crash, a new express locomotive was required and the 'Class 8' 8P Duke of Gloucester. This locomotive was a case of what might have been, the design was groundbreaking but the construction, on the other hand, was poor. Premade parts that were not in the original design were used for cost reasons, making a thirsty, hungry and poor steaming locomotive. It was only after the engine was dismantled, scrapped, rebuilt and resteamed that they showed the old engine could have been the best Pacific in the world.
The 4-6-4 was a passenger design popular in America and Europe. The New York Central Railroad built their locomotives in 1927, and called then 'Hudsons' after the river. The Milwaukee Road called theirs 'Baltics', also the European name for them. Both these names stuck. 'Hudsons' could have larger fireboxes, so could raise steam easily, thus they were great at high speed running. With most of their wheels unpowered, they struggled with gradients and also starting heavy trains, often requiring a second engine to help them start. They did however break speed records with the Milwaukee Road's class 'F6' in 1934 with 103mph, and German '05' in 1936 with 124mph. Beyond prototypes, the UK only saw 4-6-4s as tanks, one of them being the 99 ton 'Brighton Baltics' of the London, Brighton and South Coast Railway.
Eight Coupled Engines
In the UK, as engines got beyond six coupled wheels, they were primarily freight locomotives.
0-8-0 locomotives were used as freight engines and shunters. In the UK, there were few classes of them, since there were not the long trains or large yards that there were in the USA.
2-8-0s were known as the 'Consolidation' type. In the US, they supplanted the 'Mogul' as the choice of the freight locomotive, being able to pull twice as much as the 4-4-0. In the UK, they were vital freight locomotives. The GWR '2800' class was introduced in 1903 and was one of the best load haulers in the country for the remainder of the steam era. 84 of these were produced as well as 167 of the similar '2884' class. In 1948, a 2800 was tested against the latest UK freight designs and it was still successful. It was only supplanted from the heaviest freights by the BR standards. LMS's 8F 2-8-0 production run totalled to 852, it was designed by Stainer as a companion to his 'Black Five'. The War Department built 935 'Austerity' 2-8-0 for use in Britain and overseas during World War Two.
Commonly known as 'Mikados', the2-8-2s were renamed in the US after Pearl Harbour, becoming 'Mikes' or 'MacArthurs'. Over 14,000 saw service in the US, heading fast freight trains. Some 'Mikados' had the heaviest axle load ever seen on steam locomotives. In the UK, a few Mike tanks were built, but only one major type of tender engines, the Gresley 'P2s'. None of these remain as Thomson carved them up into 'Pacifics'.
2-8-4s were known as 'Berkshires'. No major classes were made in the UK, however, one featured in the 2004 comouter-animated film The Polar Express.
'Mastodons' were the 4-8-0s, a freight design that saw little service aside from tank engines in the UK.
4-8-2'Mountains' took over from 'Pacifics' in America as the trains got heavier. The only pair, Hercules and Samson that were used in the UK were narrow gauge locomotives on the Romney, Hythe and Dymchurch railway in Kent.
Twenty-four 4-8-4 ' Northerns' were built in the Vulcan Foundry in Newton-Le-Willows in the 1930s for China. 'Northerns' were used a lot in the US for passenger trains as they provided huge power, smooth running and a large firebox. However, they did suffer from having low grip on the rails.
In the UK the Ten Coupled engine were where the limits of railway infrastructure stopped engine development. Engines with more than ten coupled wheels would struggle to get around curves on the railway network, indeed, even ten coupled engines needed the flanges removed from the middle coupled wheels to get around tight curves.
In the UK 0-10-0s were known as 'Decapods', while in the US, this applied to 2-10-0s. Ill-suited to anything but slow freight, these mostly saw service around the world as shunters and as backing locomotives, used to help trains up hills. Great Eastern built the 80 ton Decapod as a fast accelerating tank, however, it would have required a lot of bridge strengthening, so production was not extended.
2-10-0s were more popular in the US than 0-10-0s as their front truck meant a lot smoother ride. In the UK only two classes were built, the War Department 150 'Austeritys' for the war effort. These were easier on the rails than their 2-8-0 sisters, essential when the track had been rapidly relayed by army engineers. 251 of the British Rail 'Standard Class 9', 9F locomotives were made. These were easily the most successful of the Standards in terms of performance, not only needing a whole new power classification for their load hauling abilities, but the surprising versatility of the design. It was hardly a shock that they help the UK record for the largest load started by a steam locomotive, as that was precisely what they were built for. What was surprising is that, despite their small wheels, they could happily deputise for a Pacific on an express, and were clocked in excess of 90mph on occasions. Drivers blamed this on a combination of smooth running and a lack of a speedometer! The last 9F and the last, 999th standard, was Evening Star built in 1960, well after BR's modernisation plan at tolled the death knoll for steam. The first 9Fs were withdrawn from service in 1964.
Nothing larger than these ran in the UK, but for completeness sake, we should say that the 2-10-2 is known as the 'Santa Fe' and the 2-10-4 as the 'Texas'.
Beyond these arrangements, most companies used forms of articulation to make large locomotives that could negotiate curves and pull large loads. This didn't stop some arrangements like the 2-12-2 Tanks used in Java, the 2-12-4 used in Bulgaria and the 4-12-2s of America9.
Nothing to do with swanky apartments or fast network connections, Duplex engines were introduced to solve some the problems involved with two cylinder engines, where the motions of the pistons and various bits and bobs were not balanced. The solution was to use four cylinders, but, in pairs. For example a two cylinder 4-8-4 engine could be made into a four cylinder 4-4-4-4, with two pistons driving the first two driving wheels and two driving the rear two. Baldwin Locomotive Works pioneered these locomotives to try and solve problems that nobody else was really bothered with, and not only that, Duplexes never really worked.
Similar in concept to the Duplexes were the Mallets, but with two major differences, the first was that they were articulated, so that the front set of driving wheels could pivot under the boiler, the other was that they were useful engines. Their articulated nature meant that they could be massive engines, without having to worry too much about getting around curves. In countries with large loading gauges10, like the USA, these beasts could come in at over 200 tons and produce well over three times the tractive effort of the most powerful UK locomotives.
True Mallets were compound engines. This means that the steam is used twice. In Mallets, the steam, at high pressure, is used first by the rear cylinders, then used at low pressure but larger cylinders connected to the front wheels. One of the biggest true Mallets was the 'Y6b', built by the Norfolk and Western railroad in the US. This 2-8-8-2 was a svelte 260 tons, with a tender of another 170 tons. Most Mallets were designed for slow moving freight trains, many over a mile in length, most of their strength was at low speeds, and few ever passed 50mph in service.
Many locomotives were built with the Mallet system of using an articulated set front driving wheels, but did without the compounding. The most famous of these is the 4-8-8-4 'Big Boy' of the Union Pacific. At 132 ft (40m) long 550 tons including tender11. While it was most effective at low speeds, it, unusually, was built to be stable at 80 mph. Few Mallets could match that.
Outside the US, Mallets were not the most popular way of making a large engine. Instead, Garratts were used. This was a design developed by Herbert William Garratt, using a boiler and firebox on a central frame and this was mounted on two separate steam engines at either end. Since these engines pivoted at either end of the boiler, it was able to round tight curves.
With few restrictions on the size of the boiler and firebox, the engines could be immensely powerful, doing away with the need for using multiple locomotives on one train. As the sets of wheels were far apart, they spread the load over a larger distance than similarly heavy normal locomotives or Mallets, so could run on lighter rails and without the need to strengthen bridges. As tank engines, they could run either way with ease, but suffered from a lack of adhesion when low on coal and water.
Garratts were able to run much quicker than Mallets around curves as they tended to swing around the inside of the curves, in contrast to the Mallets.
The first Garratts were built by Beyer, Peacock & Co of Gorton, Manchester, for the narrow gauge Tasmanian railway in 1909. One of these engines now runs on the Ffestiniog Railway in North Wales. Two thirds of all the Garratts built in the world were Beyer Garratts. The Gorton factory exported to all corners of the Earth.
In Britain a 2-8-0+0-8-2 was built for the LNER to help their coal trains get over the Woodhead route between Manchester and Sheffield. The LMS had a fleet of 33 2-6-0+0-6-2s to eliminate the need for running more than one small loco on their freight trains12. Various design weaknesses meant that these weren't that successful. On the whole British railways were not suited to the Garratts. The freight trains were short and the lines were generally flat and straight.
Elsewhere, however, they were popular. They were well suited to narrow gauge railways, where they could still be built to a vast size, produce massive power, negotiate tight corners and climb steep gradients.
Fairlies are looked on now as curiosities, funny looking engines that look like two locomotives have been stuck together. Robert F Fairlie developed a type of locomotive that had all its weight on its driven wheels, carried all its fuel on board and could run either way with no problems. The result looked like two small tank engines had got stuck together, cab to cab. Most were classed as 0-4-0+0-4-0s. Having no trailing or leading trucks, they were rough riders, some of them shaking themselves to bits. They had limited capacity to fuel and water and were cramped inside.
Again, they suited narrow gauge railways, and the Ffestiniog Railway Company is the line most associated with them. Some large Fairlies were built, with Mexico's Ferrocarril Mexicano ordered five 120-ton beasts from England around the turn of the 20th Century.