The Class K3 'Mogul' was primarily a development from the earlier acheivements by (later Sir) Nigel gresely on the GNR. His predecessor, Henry Ivatt, had already im[plemented the GNR's letter 'H' classifictaion system for the 20 engines derscribed as Class H I purchased from the Baldwin Company, USA. The letter 'H' indicated a 2-6-0 locomotive. Gresley's pioneer class of 1912 was in essence an Ivatt 0-6-0 with pony truck and outside cylinders; this received the designation Class H2. A few years later a development of this class with a larger boiler became Class H3. All were two cylinder locomotives.
Subsequently, 1920 saw the introduction of a version with even larger boiler and three cylinders described as Class H3. After the grouping they became reclassified K1, K2 and K3 by the LNER in order of their introduction. Altogether, 183 Class K3 locomotives were constructed between 1924 and 1937 at a variety of workshops in compliance with LNER standards in addition to the ten GNR version outshopped in 1920.
The emergence of Gresely's Class V2 2-6-2 locomotives in 1936 saw the end of K3 construction. The 193 examples ok K3 in traffic were used all over the former LNER system. The first was withdrawn in February 1958, and by december 1962 all were gone. Not one example survived into preservation.