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.
No. 251
J.R. Howden, „The Boys' Book of Locomotives” 
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.
Schematical drawing of No. 292
Locomotive Magazine, May 1908
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.