In December 1902, he built No. 251 by adapting the design of his "Klondike" type of 1898 and increasing the boiler diameter from 4ft 8in to 5ft 6in. The GNR classified both types as C1, but the LNER classified the large boiler variation as C1 and the small boiler variation as C2.
Ivatt strongly believed that a locomotive's success depended on its capacity to boil water, hence he gave the C1s a large (for the time) boiler and firebox. Although the boiler was large, there was not a corresponding increase in cylinder size and the C1 retained the 18.75in x 24in cylinders of the C2 Klondykes. No. 251 had the same springs as the C2 Klondike, but the heavier design resulted in an uncomfortable ride. Longer springs were quickly fitted.
Twenty more C1s were built in 1904, but the initial performance was a disappointment. The wide firebox left no space for screw reversing gear, so a lever operated mechanism was used instead. Unfortunately this was hard to operate, especially at speed. Also, drivers tended to handle the locomotives in a conservative manner due to restricting valve timings leading to a tendency to use a lot of water.

Between 1905 and 1908, sixty large-boilered C1s were built with simple expansion. These locomotives replaced the lever reversing gear with a screw reverse which was assisted by air pressure. The firebox had to be slightly re-designed so that there was enough room to fit the screw reverse. Ivatt took advantage of this needed re-design, and deepened the firebox at the front. The greater slope helped to shake coal to the front, aiding firing. Earlier locomotives needed their frames altering to take boilers with this deeper firebox. Alterations started in 1925 and were completed in 1933.
Ivatt's last development was the fitting of piston valves and Schmidt superheaters to the last ten C1s built in 1910. The addition of a superheater was matched with an increase in cylinder size from 18.75in to 20in diameter. At the time, superheating was viewed as a method to decrease the boiler pressure. Hence, the working pressure was decreased from 175psi to 150psi.
Due to poor performance, Ivatt built a four-cylinder compound version (No. 292) in March 1905. The outside cylinders drove the trailing coupled axle, whilst the inside cylinders drove the leading coupled axle. Walschaerts valve gear was used on the outside cylinders, but the inside cylinders were fitted with Stephenson valve gear. A valve was fitted beneath the smokebox, which switched the locomotive from simple to compound working. This was controlled from the cab, enabling a driver to switch between simple and compound working. The boiler was built to operate at 225psi, but actually operated at 200psi. No evidence has been found to suggest that 225psi was ever used. Between 1910 and 1912, a standard 175psi Atlantic boiler was fitted whilst repairs were being made to the original high pressure boiler. No. 292 (LNER No. 3292) was never superheated, and was withdrawn from service in August 1928. The boiler was equipped with a 24-element Robinson superheater and fitted to the four cylinder simple engine No. 3279 (see below).
The GNR directors were not satisfied either, and a second four-cylinder compound (No. 1300) was built in July 1905. This was built by Vulcan Foundry after the GNR approached five companies with broad specifications. The layout was similar to No. 292, with the high pressure cylinders on the outside, and a divided drive. The boiler had a working pressure of 200psi. Due to various troubles in the first few thousand miles, No. 1300 quickly acquired a bad reputation. Comparative trials between No. 1300, No. 292, and the simple No. 296, showed that No. 292 had the best coal consumption, but No. 294 had a lower oil consumption due to it having only two cylinders. In 1914, No. 1300 was fitted with a Robinson 22-element superheater,and the smokebox was lengthened by 9in. The working pressure is believed to have been reduced to 175psi, although the engine diagram continued to show a working pressure of 200psi. Performance was still unsatisfactory, so when No. 1300 suffered from a fractured cylinder in 1917, the opportunity was taken to rebuild it as a two-cylinder simple engine. New front-end frames were required, but the non-standard boiler was kept. No. 1300 continued to perform poorly and was withdrawn in October 1924 having only run 390,798 miles. This compares to C1 Nos. 1407-9 which all managed over 625,000 miles each over the same period.
No. 1421 was the last of Ivatt's C1 compound locomotives, and incorporated features from both Nos. 292 and 1300. No. 292's layout of cylinders and valve gear was used, but the Stephenson valve gear on the inside was replaced with Walschaerts valve gear. The slide valves were positioned slightly differently, allowing the inside cylinders to be increased in diameter to 18in. The boiler pressure was 200psi. Although no tests are known about, No. 1421 had a reputation of being a better performer than No. 292 although it also suffered from various problems. Gresley was the first to modify No. 1421, by adding a 22-element Robinson superheater in 1914. It was rebuilt a second time in 1920, as a standard C1 Atlantic with a 24-element Robinson superheater and piston valves.

Gresley succeeded Ivatt in 1911, and built no further Atlantics. However, he continued to make improvements to the C1 class, and the fact that the original 1902 Klondyke design was capable of so many improvements, delayed the need for the GN to invest in new express passenger locomotives until the early 1920s when Gresley introduced his acclaimed A1 Pacific locomotives.
Between 1914 and 1915, five saturated locomotives had Schmidt superheaters fitted. In 1914, No. 1442 was fitted with a 24-element Robinson superheater, and shortly afterwards Gresley ordered that no further Schmidt superheaters should be obtained. Robinson superheaters were gradually fitted to saturated locomotives, with the last conversion occurring in 1926. Schmidt superheaters were replaced with the Robinson type, as they wore out. The last was converted in 1927. The LNER slowly replaced 24-element Robinson superheaters with the 32-element Robinson type. The last of these replacements occurred in 1934. It is generally acknowledged that the C1s performed their best work when fitted with the 32-element Robinson superheater.