To improve the running characteristics of underground locomotives running between Moorgate Street and the South of London, Patrick Stirling introduced the 0-4-4T wheel arrangement in 1872. In contrast to other cases, where an additional axle was used to mount a bigger boiler or to increase the supplies, the new class 120 was even lighter than the previous class 126 0-4-2T. 48 had been built by 1881.

As they had to work in underground tunnels, they had a condensing apparatus. In the 1890s, many got a new domed boiler, as can be seen on the photo. When others were rebuilt around 1907, they lost their condensers and were converted into well tank locomotives. In 1905, number 533 got a special rebuild when it was converted into a crane locomotive for working at Doncaster Works.

As a development of the C1 with a small boiler, Ivatt had number 251 built in 1902, which was also an Atlantic. The most striking difference was the significantly larger boiler, which had a diameter of 5 ft 6 in instead of 4 ft 8 in. Based on the American model, its firebox stood on the frame behind the coupled axles and was significantly wider. This should serve the goal of being able to produce large amounts of steam even at the highest speeds.

The prototype had only two cylinders, slightly larger than those of the smaller Atlantic. The slide valves were soon exchanged for piston valves, improving performance. Initially 80 production pieces of this version were built between 1904 and 1908. Ten more followed in 1910, which had a factory superheater and operated with a boiler pressure of just 150 instead of 170 psi. The earlier pieces also got a superheater later.

Individual examples were built with four compound cylinders and a 200 psi pressure boiler as a trial. Number 292, which was already built in 1904, made the start. As a compound locomotive of the De Glehn type, it had external high-pressure cylinders that drove the rear coupled axle and internal low-pressure cylinders that drove the front coupled axle. The valve gear was designed in such a way that it could start up as a four-cylinder engine with simple steam expansion and then be switched to compound action. This locomotive had a boiler pressure of 200 psi and was in service until 1927.

Similar to the 292 was the 1421, also built at Doncaster in 1907. It had a similar engine, but larger low-pressure cylinders. After receiving a superheater in 1914, it was converted into a simple two-cylinder engine in 1920 and thus joined the 91 examples of the production version.

An exception was number 1300, built by the Vulcan Foundry in 1905, which like the small Atlantic had a long, narrow firebox. The arrangement of the cylinders corresponded to the other two four-cylinder locomotives, but the valve gear differed. It had an automatic starting device, which initially worked with two cylinders with simple steam expansion and automatically switched to compound operation with all four cylinders as speed increased. Like the 1421, it received a superheater in 1914 and was converted to a two-cylinder engine in 1917.

The 279, which had been part of the series production, was converted into a four-cylinder locomotives in 1915. Here, however, simple steam expansion was still used, which is why all cylinders now had a diameter of 15 instead of 19 inches. It remained in service in this form until 1938, when it received two new 20-inch diameter cylinders from the K2 class Mogul. 

Since the production version already provided very good service, no further special versions were built. Like their predecessors, they easily reached 90 mph and, in contrast to these, were able to pull up to 500 long tons at high speeds. The LNER also referred to them as class C1, while the smaller C1s now became C2s. Although one machine was equipped with a booster in 1923 to increase the tractive effort, the tractive effort of the other machines was apparently still sufficient and so this was later removed again.

Regular service on the heaviest express trains ended in the 1920s with the introduction of the A1 class Pacifics. The C1 now tended to pull lighter express trains and stood in for failed Pacifics. Especially in the latter area of operation, they often proved that they could also take over these trains without any problems and thus provided train services that were actually significantly higher than the services originally intended for them. From about 1950 they mainly only pulled regular passenger trains before they were retired between 1954 and 1960.

Shortly before the turn of the century, Henry Ivatt had the task of developing a new locomotive for the increasing loads of the most important express trains. He had to measure himself against the singles from his predecessor Patrick Stirling, who were still considered excellent and very fast, but no longer powerful enough. Here, for the first time in Great Britain, the wheel arrangement 4-4-2 was used, which originally came from the USA and was known there as “Atlantic”. The difference was that the Atlantic was developed in the US from the 4-4-0 “American” to allow for a larger firebox and smoother running, while in the UK it was a 4-2-2 “Single” with an extra coupled axle. 

The class also represented a departure from British tradition in other respects, most notably with its externally mounted cylinders. The class was initially only named after the first machine built with the number 990 and later got the designation C1. To distinguish them from the larger boiler design of 1902, the locomotives are generally known simply as “C1 (small boiler)” or “Small Atlantics”. Because of the American influence, they were also nicknamed “Klondykes” after the Klondike Gold Rush. A single example, number 271, was built in 1902 with four small simple acting cylinders.

A total of 22 pieces were made. The locomotives easily reached 90 mph, which proved difficult in everyday use on routes such as between London and Doncaster. At some speeds, it was often found that the cylinders were too small to implement the boiler's output. This led to unfavorable valve gear settings during operation, which led to high steam consumption.

In 1909, number 988 was the only one equipped with a superheater according to the Schmidt patent. The boiler pressure was reduced from 175 to 160 psi and in return the cylinder diameter was increased from 19 to 20 inches. From 1914 all other machines were converted with the Robinson superheater. From 1923, the machines were listed as class C2 by the LNER. The decommissioning began as early as 1935 and lasted until 1946. Today only number 990, which was built first, still exists, which bears the name “Henry Oakley” after the former managing director of the Great Northern. 

The obsolescence of the existing freight locomotives on the Great Northern shortly before the turn of the century faced the problem that there was no free capacity in the local locomotive factories. Neither the own workshops in Doncaster, nor the commercial British factories were able to deliver any significant number in a timely manner. So they turned to Baldwin in the USA, which designed a freight locomotive for use in England.

While in Great Britain at that time ordinary freight locomotives were normally manufactured with a 0-6-0 wheel arrangement, mainline locomotives in the USA were always designed with a leading axle or bogie. Thus the new locomotive was a lightly built Mogul, which was delivered a total of 20 times in 1899. It corresponded mainly to American design principles and got a four-axle tender with bogies. Typically British features were the placement of the sandbox under the running boards and a British-design chimney.

During operation, a somewhat high coal consumption and a very high consumption of lubricating oil were noticed. Despite the leading axle, their running was considered unsteady and due to the light construction, the continuous steaming performance did not have any significant advantages compared to earlier freight locomotives. Even the experimental use with London suburban trains from King's Cross did not bring convincing results. Thus, the decommissioning took place between 1909 and 1915, when enough freight locomotives from indigenous production were available.

The Great Northern class H4 was a 2-6-0 mixed-traffic locomotive designed by Sir Nigel Gresley. Its basis was the H3, but now it got three instead of two cylinders, being the first production locomotive to use Gresley's conjugated valve gear. All three cylinders worked on the second driving axle. With a huge boiler being six feet in diameter, the axle loading became somewhat high for some lines.

The first ten locomotives were built in 1920 at Doncaster and the LNER adopted this class as a standard design. With additional batches built at Doncaster, Darlington and three commercial manufacturers, 193 had been completed by 1937. They were designated class K3 by the LNER. Due to their rhythm of the exhaust beat and the uneven gyratory movements, they earned the nickname “Jazzers”.

Nevertheless, they were able to run at up to 75 mph (121 km/h) and were also used to haul express trains when needed. So they could be rated as being very successful, although Thompson rebuilt one in 1945 into the two-cylinder K5. All were withdrawn between 1959 and 1962 with none being preserved. The A1 steam trust once announced that they might build a new K3, but as of today no new news can be found about that topic.

To haul heavy coal trains from Peterborough to London, Ivatt developed the class K, which was soon dubbed the “Long Tom” because of its longer boiler compared to the six-coupled locomotives. The dimensions of this boiler roughly corresponded to the class 990 or C1 (small boiler) Atlantics. It could generate enough power to haul 60 full wagons of coal. This corresponded to a train weight of 1,052 tons compared to 681 tons for the older, six-coupled freight locomotives. With the cylinder dimensions required for this, it was still possible to arrange the cylinders within the frame. From 1901 a total of 55 engines were built.

Although the engines could handle the required train weights without any problems, a high coal consumption was noticeable. Therefore, from 1908 almost all engines were superheated, with two different variants being created. The first one, later designated by the LNER as the Q1s class, retained the original cylinder dimensions and slide valves. The others, which later became LNER class Q2, used cylinders with a diameter of 21 instead of 20 inches and piston valves instead of slide valves. Both variants had Schmidt-type superheaters and both had the fully automatic Klinger-type lubrication required for superheated operation.

In 1913 number 445 was fitted with the Doncaster superheater developed by Gresley. No other engines with this superheater followed. From 1923, it was listed as class Q3 by the LNER due to its different power characteristics. In 1914, number 420 was fitted with a significantly larger boiler. Since the other boilers of this type were finally used for the 2-6-0 machines of class K2, the 420 remained a one-off.

In the 1920's the superheaters were replaced with the Robinson type which were standard on the LNER. With the introduction of the 2-8-0 locomotives of classes O1 and O2, the locomotives were no longer needed for transporting coal to London and were increasingly used in more northerly areas. During this time they were nicknamed “Sea Pigs” because of their high water consumption compared to other locomotives. From 1926 the retirement began, whereby the few locomotives with saturated boilers were affected first. By 1935 all Q1s and Q2s were gone and the only Q3 lasted until 1937.

The L1 was a Great Northern Railway tank locomotive designed by Henry Ivatt for running commuter trains on the Metropolitan City Lines in London. Although four coupled axles and a driving wheel diameter of only 4 feet 8 inches looked more like a freight locomotive, this design was chosen to achieve the fastest possible acceleration. The boiler and the cylinders were largely the same as the eight-coupled class K goods locomotives. Since the area of operation was in the city and partly in tunnels, a facility for condensing the steam was used. This was recognizable by the pipes from the smokebox to the water tanks.

At first only a prototype was built and it quickly became apparent that the axle load was too high. This can be explained by the fact that the weight of the large boiler was additionally increased by the water reserves and the condensation device. That is why the ten other production engines were created with a slightly smaller boiler, cylinders adapted to it and reduced water supplies. These eleven engines were used in passenger service as planned, but they do not seem to have performed particularly well. While they demonstrated the ability for very rapid acceleration to 30 mph, they did not significantly exceed 40 mph. 

As the locomotives had great pulling power, 30 more were built in 1905 and 1906 for use on freight trains in West Riding of Yorkshire. In 1907 the eleven others from London were also transferred there. Spread over the years from 1909 to 1926, the locomotives were given a larger boiler that corresponded to the original plans. A superheater was installed on seven and the boiler pressure was reduced from 175 to 170 psi. Locomotive number 131 was experimentally converted to oil firing. From 1923 they became class R1 at the LNER. From 1927 the first engines were retired. During this time there was a proposal to convert the locos to diesel powered compressed air locos. A 400 hp diesel engine would have filled a tank with compressed air, which would then have acted on the existing cylinders. However, these plans were not implemented and in 1934 the last engine was retired.

After the L1, Ivatt developed another tank locomotive for the GNR for use on the city lines of London. In order to be better suited for higher speeds, there were only three coupled axles with 5 ft 8 in wheels. As with the L1, the first example of the N1 was too heavy for the Metropolitan Lines. In the following examples, the axle load on the coupled axles was reduced by placing more weight on the trailing axle. This was done by moving them further back and shortening the front water tanks and accommodating more water behind the cab. A total of 56 units were produced by 1912, all but six of which were again equipped with a condensation device. The latter six were used in the West Riding of Yorkshire.

Initially, most N1s remained in service in London. During World War I, two were sold to the War Office, one of which was used to build an armored train. In the years that followed, superheaters of the Schmidt type were installed in eleven units. From 1920 Gresleys N2s replaced the N1s in passenger train service. The N1s continued to operate in north London, but mostly to haul empty passenger coaches and freight trains. After the locomotives had come to the LNER, the remaining ones were fitted with Robinson type superheaters and the eleven that had already been converted were also refitted. Gradually more locomotives came to Yorkshire where the condensing facilities were removed. Most ended up with British Railways, the last being retired in 1959.

As a replacement for Ivatt's N1, Nigel Gresley developed the N2, which was again a tank locomotive with a 0-6-2T wheel arrangement and a driver diameter of 5 feet 8 inches. They were superheated from the factory and weighed a little more than the N1. Because the valves were mounted on top of the inside cylinders, the boiler had to be mounted relatively high. This necessitated a flat chimney to fit the Metropolitan Line's loading gauge. The first 60 examples were built in 1920 and 1921 and operated in north London, with Kings Cross as their home station. They always had enough to do, as new residential areas were constantly being developed in the north and these were connected via Kings Cross. Typical wagon material consisted of the four-part passenger wagons known as “quad-art”, which were connected via Jakobs bogies and from which one or two sets were usually pulled.

After the British railway administrations were grouped, the LNER had a further 47 units built between 1925 and 1929. These consisted of different versions, some of which did not have condensation devices and had different braking systems. These were used in Scotland on the Edinburgh, Glasgow and Dundee suburban railways. Speed limits sometimes had to be introduced on winding routes because the engines had a high center of gravity. Over time there have been several conversions in which various forms of superheaters have been installed. All examples built were still in use when British Railways was formed in 1948. Gradually, diesel railcars took over the tasks of the N2, so that they gradually disappeared between 1955 and 1962. One example is still available today and is roadworthy. It has had the GNR's apple-green livery since 2009.

The O1 was designed by Sir Nigel Gresley for the Great Northern Railway to reduce the number of heavy coal trains between Grantham and London with more powerful locomotives. So far, the classes Q1 and Q2 with the 0-8-0 wheel arrangement had been used for this task and the required larger boiler was to be made possible by means of an additional axle. Since there were many diverging points to be run over when entering London, the additional axle was added as a leading axle to improve the running characteristics. The number of four coupled axles remained the same.

Gresley derived the larger boiler from the class C1 Atlantic express locomotives that his predecessor Ivatt had developed. As with the 2-6-0 locomotives of the K1 and K2 classes, Gresley arranged the cylinders on the outside in order to be able to increase their dimensions. The very first engine showed its capabilities, hauling a train of 78 two-axle coal wagons and two 20-ton brake vans at speeds of up to 27.5 mph. Their predecessors had been limited to 60 cars.

The first five locomotives were built in Doncaster in 1914, but the second order of 15 could not be completed due to the war. However, after the end of the war, the completed parts could be taken to the North British Locomotive Company and delivered as finished locomotives in 1918 and 1919. A development of the O1 was the O2 with three cylinders. As with all GNR stock, the O1 were taken over by the LNER in 1923 and were renamed O3 in 1944, as another locomotive called the O1 by Thompson was introduced at that time. When British Railways was founded in 1948, there were still 17 examples, all of which were retired by 1952.