The K1 was a mixed traffic locomotive designed for the LNER, but was only delivered directly to British Railways in 1949 and 1950. Since Edward Thompson was a fan of two-cylinder locomotives, he rebuilt a K4 from three to two cylinders in 1945. Since the new cylinders were only slightly larger than the old ones, he increased the boiler pressure to keep the loss of tractive power to a minimum. This prototype was designated K1/1 and was considered a success.

Series production of 70 K1s did not begin until 1949, when Arthur Peppercorn was chief engineer of the Eastern and North Eastern Region of British Railways. Compared to the prototype, he had made some changes, which included improvements to the firebox and the suspension of the leading axle. He also now used a tender with a water capacity of 4,200 gallons instead of 3,500.

The K1s mostly remained in service in the area of the former LNER. With a maximum forward speed of 50 mph and reverse 45 mph, they were used on all types of trains, including express trains. Their withdrawal took place between 1962 and 1967. Only the 62005, which has been operational again since 1975, has been preserved. It wore LNER livery most of the time with the imaginary number 2005, which is historically incorrect.