The Great Western Railway 4900 Class or Hall Class is a class of 4-6-0 mixed-traffic steam locomotives designed by Charles Collett for the Great Western Railway. A total of 259 were built at Swindon Works, numbered 4900–4999, 5900–5999 and 6900–6958. The LMS Stanier Class 5 4-6-0 and LNER Thompson Class B1 both drew heavily on design features of the Hall Class. After nationalisation in 1948, British Railways gave them the power classification 5MT.
 By the end of 1923 the Great Western Railway (GWR) was well served with express passenger locomotives of the Saint and Star classes and had recently introduced the Castle Class. However, the mixed-traffic 2-6-0 locomotives of the 4300 Class were beginning to struggle with the increasing loads. George Jackson Churchward had recognised this with the introduction of the 4700 class 2-8-0 with 5 ft 8 in (1.727 m) driving wheels, intended for express goods and relief passenger trains. However, Charles Collett preferred the idea of a Saint Class with smaller wheels to undertake these duties as this would provide a leading bogie. He therefore rebuilt number 2925 Saint Martin with 6 ft (1.829 m) driving wheels.
 The prototype of the new class was rebuilt in 1924 and the cylinders were realigned in relation to the driving axle and a more modern 'Castle'-type cab was fitted. Saint Martin emerged from Swindon Works in 1924 and embarked on three years of trials. During this period Collett introduced other modifications such as changing the pitch of the taper boiler and adding outside steam pipes.
 After extensive trials during 1925–1927, Collett was satisfied with the performance of his prototype, subject to minor amendments and placed an order for eighty more with Swindon works (Lot 254) in 1928. The prototype was renumbered 4900 in December 1928 and the new locomotives were numbered 4901-80 and appeared at regular intervals until February 1930. They were named after English and Welsh country houses with 'Hall' in their titles and so became known as the 'Hall Class'.
 They differed little from the prototype; the bogie wheel diameter had been reduced by two inches from 3 ft 2 in (0.965 m) to 3 ft 0 in (0.914 m) and the valve setting amended to give an increased travel of 7.5 in (191 mm). The overall weight of the locomotive had increased by 2 long tons 10 cwt (5,600 lb or 2.5 t) to 75 long tons 0 cwt (168,000 lb or 76.2 t) but a tractive effort of 27,275 lbf (121.33 kN) compared favourably with the 24,935 lbf (110.92 kN) of the 'Saint'. The original locomotives were built with Churchward 3,500 imp gal (16,000 L; 4,200 US gal) tenders but after 4958 Collett's larger 4,000 imp gal (18,000 L; 4,800 US gal) types became standard although a few later locomotives were fitted with smaller tenders if these were available as they entered service.
 The first fourteen examples were despatched to the arduous proving grounds of the Cornish Main Line. They were so successful here and elsewhere on the GWR system that by the time the first production batch had been completed a further twenty were on order (Lot 268, 4981–99 and 5900). Further orders followed throughout the 1930s and early 1940s. By 1935, 150 were in service and the 259th and last Hall, No. 6958 Oxburgh Hall, was delivered in 1943. Thereafter further deliveries were of the '6959 Modified Hall' class.
 Eleven Hall class locomotives were converted to oil-firing in the period 1946–1950. While in this condition they were renumbered into the 3900 series. When the oil-firing was removed, they reverted to their old numbers.
 As indicated by their continuing production, the Hall class proved to be very successful in a variety of different roles from goods work to passenger services, although barred from several cross-country and branch lines because of their red weight classification. According to Peter Herring, 'they were the first true mixed traffic locomotives, and as such precursors of the Stanier 'Black Five', Thompson B1 and BR Standard 5MT 4-6-0.' (However, while they were forerunners of these highly successful and numerous 4-6-0 types, there were several successful 2-6-0 and 4-6-0 ‘mixed traffic’ types on the GWR and other British railways before them, - not least the GWR 4300 Class they were designed to replace.)
 Although the GWR had been at the forefront of British locomotive development between 1900 and 1930, the 1930s saw a degree of complacency at Swindon reflected in the fact that the design had largely originated in the 1900s and had not fundamentally changed since the mid-1920s. Collett was replaced by Frederick Hawksworth in 1941 who created a modified version of the design, known as the Modified Hall Class. These continued to be produced by British Railways until 1950, by which time there were a further seventy-one locomotives.
 All but one of the original Collett Halls survived until nationalisation in 1948, the exception being 4911 Bowden Hall. Withdrawals began in 1959 with the prototype Saint Martin. Its accumulated mileage, both in its original form and rebuilt form, was a remarkable 2,092,500 miles. Further withdrawals of the production series took place during the 1960s with the final 9 members of the class being withdrawn in December 1965.
 By 1965, the last Hall had been withdrawn from the Western Region without a single example entering the National Collection. Despite this, 11 of the class did survive into preservation, all being rescued from Barry Island Scrapyard. The first member of the class to be rescued, and the 10th departure from Barry, was 4965 Rood Ashton Hall, which left in October 1970. The engine was at this point assumed to be 4983 Albert Hall, but in 1998, the restoration team at the Birmingham Railway Museum discovered its true identity to be that of 4965.  The last unmodified Hall to leave Barry, and the 187th departure, was 5967 Bickmarsh Hall in August 1987. 
 Seven of the class have run in preservation, with all six of the UK-based engines having operated on the main line: 4930 Hagley Hall, 4936 Kinlet Hall, 4953 Pitchford Hall, 4965 Rood Ashton Hall, 5900 Hinderton Hall and 5972 Olton Hall. 5972 Olton Hall gained fame as 'Hogwarts Castle', the locomotive used in the Harry Potter film series. 4920 Dumbleton Hall has operated in preservation, but is now on static display as part of the new Harry Potter attraction which opened in Tokyo in 2023.
 As of September 2022 two Halls were operational but neither mainline certified. 4936 Kinlet Hall is undergoing a Network Rail standard overhaul at Tyseley Locomotive Works.
 Of those engines which have not run in preservation, 4942 Maindy Hall has been converted back to a GWR Saint Class (2999 Lady of Legend), 4979 Wootton Hall is undergoing restoration at the Ribble Steam Railway with work currently focusing on the engine's tender, 5952 Cogan Hall is under cosmetic restoration at Tyseley Locomotive Works, with a small number of parts being used in the construction of 6880 Betton Grange and 5967 Bickmarsh Hall is undergoing restoration at the Northampton & Lamport Railway.
 Following cosmetic makeover at Carnforth MPD, including repaint into Hogwarts Railways Crimson, was exported from Southampton to Tokyo, Japan in December 2021.
On long term loan to Warner Bros.
 Undergoing cosmetic restoration for static display at Tyseley Loco Works, full restoration to commence following the completion of 6880.