The K1 class was introduced in 1949 for service on the Highland lines. The engines were a further development of the earlier K4 locomotives, Thompson took the opportunity to simplify the engines to a 2-cylinder arrangement, however much of the work on this class was undertaken by A H Peppercorn before he took over from Thomson as chief engineer.
The 2-6-0 arrangement was used to provide a powerful, modern locomotive with the extra traction available from the 3 driving axles but within the weight and length constraits applicable to the Mallaig line bridges and turntables. One example of the class is preserved and has been operated regularly on the scheduled summer steam excursions between Fort William and Mallaig.