To haul heavy coal trains from Peterborough to London, Ivatt developed the K class, 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 long tons compared to 681 long tons for the previous, 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 made.

Although the engines could handle the required train weights without any problems, the high consumption of coal 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 piston slides. The other, which later became the LNER's Q2 class, 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 steam 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 the different performance 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 the K2 class, the 420 remained a one-off.

No. 420 with bigger boiler

Locomotive Magazine, December 1918

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 engines 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 wet steam boilers were affected first. By 1935 all Q1s and Q2s were gone and the only Q3 lasted until 1937.