31806 is the only survivor of the U class locomotives that was originally built as a K class 2-6-4T. It was originally built at Brighton as A806 River Torridge in 1926 but following the accident at Sevenoaks was rebuilt as a U class 2-6-0 at Brighton in 1928.
After entering traffic at Guildford it soon afterwards moved to Nine Elms early in 1929. It remained at Nine Elms until the late 1930s when it moved to Basingstoke and then Guildford by the beginning of 1945.
BR motive power depot allocations since 1948.
| Date Arrived | Depot |
| January 1948 | Guildford |
| August 1949 | Stewarts Lane |
| August 1949 | Nine Elms |
| October 1949 | Faversham |
| October 1954 | Nine Elms |
| July 1955 | Basingstoke |
| February 1963 | Guildford |
In November 1957 the engine received a new front-end section to its frames during a heavy overhaul.
After withdrawal from service in January 1964, having covered 970,103 miles, it was stored at Fratton before going to the scrapyard at Barry belonging to Woodham Brothers. It arrived at Barry in June 1964.
It was purchased in 1975 by John Bunch in 1975 and left Barry in October 1976 and moved to the Mid Hants Railway at Alresford where its restoration started in 1979 with the result that it was restored and returned to service in April 1981.
31806 ran on the Mid Hants Railway for a decade until the boiler certificate expired after which it remained out of service for many years. It finally returned to steam again on the Mid Hants Railway in 2011 and has a boiler certificate valid until 2021.
In 2014 the owner, John Bunch, moved it to the Swanage Railway who agreed a 25 year lease for the locomotive.
Operational SR Maunsell U class 2-6-0 No. 31806 is to arrive in time to start hauling trains at Swanage during August. The Brighton-built ‘U-boat’ as the class were nicknamed, ran on the Swanage branch in the 1950s under BR auspices. It has visited the line in the preservation era, most recently for the Swanage Railway’s April 5-6 2014 Spring Steam Gala. It has seven years remaining on its boiler ticket.
The locomotive is currently undergoing a bottom end overhaul at Herston works. Southern Locomotives Ltd have been contracted to carry out an overhaul of the tender frames which will included modifications to the pipework to bring it in line with the original drawings. The locomotive has been fitted out for main line electronic equipment with a view to the engine being able to work into Wareham at some stage in the future.
In April 2018 the locomotive hauled the Yeovil to Swanage segment of the Great Britain Xl. This was its inaugural main line running.
In May 2019 the locomotive was lifted off its wheels at Herston works. This was to give the axleboxes some attention.
Late in December 2020 it was announced that a decision had been taken to fit the boiler from 31874 to 31806 rather than to 31874. The logic for this being that the boiler certificate for 31806 expires in April 2021 and the Swanage Railway needs one of the locomotives to be in service early in 2021. The work required to complete the overhaul of 31874 meant that this locomotive could not be in service on this timescale. By fitting the boiler from 31874 onto 31806 in January 2021 the latter locomotive can be in traffic on the Swanage Railway in time for the 2021 season.
| Home Base | Current Status | Owner |
| Swanage Railway | Under overhaul | Leased to the Swanage Railway for twenty five years by the owner John Bunch in 2014 |
- 31618 (SR A618, SR 1618 & BR 31618)
- 31625 (SR A625, SR 1625 & BR 31625)
- 31638 (SR A638, SR 1638 & BR 31638)
Preserved British Steam Locomotives 