Inspired by Thompson’s rebuilt Gresley K4 in which it was reclassified as a K1/1, the only locomotive in that particular Class, A. H. Peppercorn took advantage of Thompson’s retirement in 1946 to develop the design into the K1 Class.

The K1 Class, which was made up of 70 locomotives, were all built in Glasgow at the North British Locomotive Company’s Park Works over a 10 month period between May 1949 and March 1950. As was the North British Locomotive Company’s all of the Class were sent to Eastfield shed, Glasgow for running in before being sent to England where they were used for both freight and express passenger workings.  Thirty of the Class were sent to the Eastern Region while the remaining forty were dispatched to the North Eastern Region. Many of the North Eastern K1s were often seen as far North as Edinburgh and early on in their working life were regularly used on the West Highland line. A significant number of those allocated to the Eastern Region were based at March.