Richard Maunsell's K class of 2-6-4T tank engines had stability problems and following the Sevenoaks Rail Disaster a solution was required. Two main solutions were proposed one was relaying vast quanities of trackwork, the other was rebuilding them. Plans for a new passenger mogul were already underway and were brought forward to convert theK class into the new U class.
One of these converted locomotives survive today in the form of 31806, which is based on the Swanage Railway.
In the end the class numbered 50 locomotives. Four in total survive today. Along with 31806, which is owned by John Bunch, he also owns 31265 which is currently awaiting overhaul at Swanage. The other two survivors are based at the Bluebell and are currently out of traffic in the form of 1618 and 1638.