31874 was designed by Maunsell at Brighton Works but constructed at Ashford Works in 1925 built from parts manufactured at Woolwich as a post First World War initiative which were sold for £4,000 each. It entered service as Southern Railway A874 – the A denoting Ashford.
It was initially based at Bricklayers Arms depot in where it was mainly used on main line goods duties. In 1935 it went to the newly opened shed at Norwood Junction to haul trial goods workings before returning to Bricklayers Arms. It remained at Bricklayers Arms for several years apart from a period from October 1938 to January 1939 when it was based at Higher Green.
Following the reallocations of locomotives to cater for traffic during the Second World War 31874 was moved to Guildford in March 1944. After the War Department (WD) engines were shipped abroad for war service moved to Easleigh in 1945. Amongst other jobs it was used for at Eastleigh it hauled WD locomotives to Southampton Docks for shipment overseas.
In January 1948 it was moved Exmouth Junction.
BR motive power depot allocations since 1948.
| Date Arrived | Depot |
| January 1948 | Exmouth Junction |
| May 1948 | Plymouth Friary |
| October 1948 | Exmouth Junction |
| November 1950 | Salisbury |
| June 1951 | Hither Green |
| February 1961 | Exmouth Junction |
In May 1957 31874 received new front-end frames as part of a major overhaul.
Due to the electrification of the lines through Kent and the arrival of main line diesel locomotives 31874 was transferred to Exmouth Junction in February 1961.
When 31874 was withdrawn from service at Exmouth Junction in March 1964 it was withdrawn from a Western Region depot as Exmouth Junction was transferred to Western Region control in September 1963.
It was sent to Woodham Brothers at Barry for scrap – the only class N locomotive to be sent to Barry where it arrived in June 1964. When withdrawn it had completed 966,499 miles in service. 31874 remained at Barry for almost ten years before leaving Barry in March 1974 after being purchased by John Bunch and transported to the Mid Hants Railway.
Trial steamings were undertaken during 1976 and four weeks of hauling test trains began in April 1977. At the end of April 1977 the locomotive was given the name Aznar Line after the shipping company that helped to pay for the locomotives move from the Barry scrapyard. After this it hauled the first passenger train on the re-opened Mid Hants Railway.
In January 1979 the name-plates were removed and new ones fitted bearing the name Brian Fisk fitted. Brian Fisk was the late husband of Yvonne Fisk, whose company had contributed significant sums of money towards the cost of restoring the locomotive.
31874 was taken out of traffic in 1981 for overhaul after which it was back in service in 1982.
It was again withdrawn from service in 1997, due to problems that require firebox reconstruction, after which it remained out of service at Ropley until 2014 when it was moved to the Swanage Railway.
In August 2014 the boiler was lifted off the frames and sent to Adam Dalgleish Engineering Ltd in Stockton where it is undergoing extensive repairs.
It has also been found that the poor state of the tender means that a new set of frames for the tender needs to be produced.
The overhaul of the locomotive at the Swanage Railway is being done to main line standards so as not to preclude main line running in future.
Once overhauled 31874 will remain on the Swanage Railway as a result of 25 year lease agreement between the owner and railway.
In May 2016 it was announced that the overhaul which the locomotive was undergoing work include certification for operation on the main line.
The main line certification was possible because of a grant from the Department of Transport and Rail Safety and Standards Board. The full grant of £75,000 is part of a £100,000 project by Swanage Railway to produce a main line capable train consisting of 31874 and five MK1 coaches. Subject to track access agreements, pathing and regulatory items being in place, this will enable Swanage Railway to operate Steam over its main line connection to Wareham. Swanage Railway may also at its discretion consider approaches from other operators for use of the locomotive.
The boiler was lifted from the frames in August 2014 and taken to the Stockton works of Adam Dalgleish Engineering Ltd. along with the tender. The remainder of the locomotive was lifted from its wheels and taken to Swanage Railway’s Herston Works.
At the start of December 2016 the locomotive was reunited with its driving wheels with the assistance of Sonic Rail Services Ltd.
It was hoped to have the locomotive back in operation in 2017 and be ready for main line running in late in 2017 or in 2018.
By the end of 2017 it was anticipated that the locomotive would be in traffic on the Swanage Railway during 2018. The key factor was said to be when the boiler was returned from Northern Steam Engineering.
The boiler was said to be nearing completion of its overhaul in July 2019.
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
The frames and wheels of 31874 are complete at Herston works near Swanage.
| Home Base | Current Status | Owner |
| Swanage Railway | Under overhaul | John Bunch |
Preserved British Steam Locomotives 