LB&SCR B4 class

The performance of Robert Billinton's B2 class 4-4-0 locomotives of 1895-1897 had proved insufficient and they were unable to replace the earlier Stroudley B1 class 0-4-2 on the heaviest London to Brighton express trains. Billinton therefore sought authority to construct 25 larger and more powerful 4-4-0 B4 class locomotives. The first two, Nos. 52 and 53, were completed at Brighton works between December 1899 and January 1900, both of which performed well and showed the new design was sound. However, during the spring of 1900 a backlog of repair work at Brighton meant that the third (No. 54) was not completed until May of that year. The railway therefore approached Sharp, Stewart and Company to supply twenty-five further examples over the next twelve months. These were all delivered between June and October 1901. By 1901 Brighton had overcome the backlog of repair work and five further locomotive boilers were ordered from Sharp, Stewart and Company to be used on additional locomotives to be built at Brighton between June and September 1902.

The B4 class successfully hauled the heaviest express trains on the London, Brighton and South Coast Railway until around 1912 when they were gradually replaced by the larger H1, H2, J1 and J2 classes. Thereafter they were regularly used on slower and lighter services. According to O. S. Nock, the B4 class "were among the finest passenger locomotives of their day".

In 1918, No. 46 was rebuilt by Lawson Billinton with a new boiler including a Robinson superheater, but the resulting locomotive was not tested before he decided to rebuild other members of the class, using his K class superheated boiler. The rebuilt locomotives were classified B4X, but since the original frames could not be used (as the K class firebox was too long to fit between the axles, new frames, and new piston valve cylinders) they were virtually new engines. However the original motion and motion plate was retained to save costs, which meant that the piston valves were in the constricted space below the cylinders at an angle to the axis of the cylinders. Only 8 in (200 mm)-diameter valves could be fitted in, and combined with the restricted exhaust arrangement ensured they could not make full use of the K class boiler.

Acceleration from stops was very slow and they could only reach speeds up to 70 mph (110 km/h) while newly outshopped and with great difficulty in places, where the Atlantics, J class and Baltics could reach 80 mph (130 km/h). Twelve members of the class were 'rebuilt' between August 1922 and January 1924, but further modifications were deferred by Southern Railway CME Richard Maunsell when it became clear their performance was not satisfactory. When Harold Holcroft was tasked by Maunsell to report on the post grouping loco stock, he found that the B4x were very expensive compared to the SECR rebuilds (D1/E1) and far less capable. It wasn't until 1929 that they could be transferred from express work to secondary duties.

The B4 and B4x classes remained in service, but thirteen examples were withdrawn between 1934 and 1939. Withdrawals paused with the outbreak of World War II and six B4s and twelve B4Xs passed to British Railways in 1948. All had been withdrawn and by 1951.