Although originated from the K class the U class also had many parts in common with the earlier N class and their appearance was similar. The U class had six feet diameter drivers as befitting its passenger status, and a boiler pressure of 200lb psi providing a tractive effort of 23,866 lb at 85 percent of boiler pressure. The total weight of engine and tender in working condition was 102 tons 16 cwt.

Engines were built or rebuilt as follows:

A610-A619 1928 at Brighton
A790-A796 rebuilt 1928 at Eastleigh
A797-A802 and A805 rebuilt 1928 at Ashford
A803-A804 and A806-A809 rebuilt 1928 at Brighton
A620-A629 1929 at Ashford
A630-A639 1931 at Ashford

The K class conversions were identifiable by a lower running plate leading to larger splashers, double spectacles to the cab front and cab side cutaways which extended into the curved part of the roof. They also retained their original numbers although they lost their names.

A three cylindered version of the class - the U1 class - was also built between 1928 to 1931 with one, A890, being a conversion of the solitary three cylindered K1 class 2-6-4T.

Throughout the life of the class there were detail differences underlining the need for modellers to obtain a good photograph of a specific engine. As originally built the class had piston tail rods, tapered chimneys and smokebox snifting valves. As with the N class the tail rods and snifting valves were removed and new chimneys of the U1 pattern fitted. Later builds of the class had a flatter top to their dome covers. Short smoke deflectors were fitted, as with all Maunsell Moguls, from 1933. The class either hauled 3500 gallon capacity tenders with straight sides or larger 4000 gallon tenders with turned in tops to their sides. From 1955 twenty three of the class were fitted with varying combinations of new cylinders with outside steam pipes and a BR pattern chimney. A fewer number also received new frames with a gentler curve between the buffer beam and smokebox.

N°A629 was for two years fitted with an experimental pulverised fuel burner of German design. This was not a success and was abandoned after it was found there was no fuel saving and that powdered coal was potentially explosive. In 1948 two of the class, 1625 and 1797, were experimentally operated as oil burners. Further members of the class were designated for conversion before the experiment was abandoned.

Although perhaps an unsung and unglamorous class the Us performed well doing exactly what they were designed to do. The first withdrawal was in 1962 with the final withdrawals being in 1966.

Four Us have been preserved though all are not currently in working condition. 1618 and 1638 are owned by the Maunsell Locomotive Society and are based at the Bluebell Railway, with 31625 and 31806 based on the Mid-Hants Railway.