The class represented the penultimate stage in the development of the Southern Railway's 2-6-0 "family", which improved upon the basic principles established by GWR Chief Mechanical Engineer (CME) George Jackson Churchward for Great Western Railway (GWR) locomotives.

[6] It was intended to replace several elderly 4-4-0 classes within the former SECR's running fleet, and attempted to standardise and ease maintenance of locomotives by sharing parts with other Maunsell designs.

[7] The rebuilt locomotives were given a tender and a re-designed cab that bore a strong resemblance to those featured on designs by Henry Fowler.

[7] The rapid turnaround was achieved as design work was already in place, and the rebuilding of existing locomotives was cheaper than building from scratch.

The rebuilds lost their names because of the bad publicity attached to the "River" class after the 1927 crash, and the heavily damaged No.

As a result of Harold Holcroft's position as one of Maunsell's assistants, the new-builds also displayed the Churchward GWR 4300 Class influence.

[12] The first of the production batch emerged from Brighton works in August 1928 and featured a tapered chimney and smokebox snifting valves, both of which were used on the K and N class locomotives.

[1] The production locomotives had a slightly different profile to the K class rebuilds and featured left-hand drive to improve the visibility of signals from the driver's side of the footplate.

[8] In common with the N, N1 and K class locomotives, the Midland Railway influence of Clayton showed in the placement of locomotive fittings on the production batch, as the water top-feed into the boiler was located inside a dome, whilst the cab area was a modified version of those on the 0-6-0s of Henry Fowler and the K class rebuilds.

[7] However, all members of the U class were to vary from GWR practice, as the superheating surface area was increased, and all were equipped with outside Walschaerts valve gear.

[10] The U class was a reliable and economical design more than capable of attaining speeds in excess of 70 mph (110 km/h) as a result of being fitted with long-travel valves.

[13] They had high capacity, tapered boilers which promoted free steaming, and 6 ft (1.83 m) driving wheels which allowed fast running over long distances.

A629 was fitted with an experimental pulverised fuel burner of German design; the experiment was terminated when a minor explosion was caused by the powdered coal coming into contact with sparks thrown from the blastpipe.

The experiment also proved that pulverised coal was a false economy, as much of the fine dust was sucked through the chimney without combustion taking place.

[13] A few members of the class were given replacement frames at overhaul with a shallower curve between the front buffer beam and smokebox.

[1] The favoured form of 2-6-0 motive power west of Exeter was the smaller-wheeled N class, while heavier passenger work was allocated to Bulleid's Unrebuilt Light Pacifics, which were within the weight restrictions imposed in this area.

[5] The entire class was absorbed by British Railways in 1948, and was given the Power classification 4P3F, denoting a mixed traffic locomotive.