 The seventy K1s were intended to be split between the North Eastern Region of British Railways and the Eastern Region of British Railways.LNER Class K4 number 3445 "MacCailin Mor" was rebuilt in 1945 as a two-cylinder prototype of the K1 class, designated K1/1.Thompson entrusted the rebuilding of No.3445 as a two-cylinder Mogul to his principal assistant Arthur Peppercorn.