The Southern Railway class U were 2-6-0 passenger locomotives which were created by Maunsell when the class K 2-6-4T were found out to be prone to derailment. So the 20 first locomotives of the class U were rebuilt from the class K at Brighton and Eastleigh, while 30 more were built between 1928 and 1931 at Brighton and Ashford. They were influenced by the GWR class 4300 and SECR class K and had a tapered boiler.

With the class U, it was possible to withdraw some old 4-4-0 locomotives. Due to their low axle load, they could appear suddenly in nearly all corners of the Southern network, so they earned the nickname “U-boats”. The long-travel valves easily allowed speeds of 70 mph or more. British Railways got all 50 in 1948 and withdrew them between 1962 and 1966. Four were preserved and operational again at some time, but today only 31806 is operational.