Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of four leading wheels on two axles, usually in a leading truck or bogie, eight powered and coupled driving wheels on four axles and no trailing wheels. In North America and in some other countries the type was usually known as the Twelve-wheeler. On a Camel locomotive the cab was mounted atop the boiler, unlike the later Camelback locomotive whose cab straddled the boiler which first appeared around 1877.
thumb|CPR no. 229, the Mastodon of 1882
The nickname Mastodon is often mistakenly used to describe the wheel arrangement and was derived from the unofficial name of the first locomotive of the Central Pacific Railroad in the United States, the wood-fired CPR no. 229, which was designed and built in 1882 by the railroad's master mechanic, Andrew Jackson (A.J.) Stevens, at the railroad's Sacramento works in California. Two of these locomotives are preserved.
thumb|left|South Australian Railways T class
A new class of locomotive, the T class, designed in South Australia for use on the narrow gauge gauge system of the South Australian Railways, was introduced in 1903. It proved to be a suitable workhorse and by 1917 there were 78 locomotives in the class. In 1921 and 1922, the Tasmanian Government also purchased six of these narrow gauge South Australian locomotives and, during 1922 and 1923, five of the class were converted to gauge for use on the broad gauge system of the South Australian. These were converted back to narrow gauge in 1949. During the Second World War, the Commonwealth Railways obtained four of these South Australian narrow gauge locomotives on loan. Several of these locomotives are preserved.
France
thumb|left|PLM , without its tender
In France, the wheel arrangement was used on two locomotive classes. The first was in 1907 by the Chemins de fer de Paris à Lyon et à la Méditerranée (PLM). These locomotives were intended for goods trains, as well as passenger trains on the more difficult routes. They were Baudry type compound locomotives, similar to the De Glehn type, but with their low pressure cylinders set at 60% cut-off. All of them originally used saturated steam, but some were later equipped with superheaters while all the others were provided with feedwater heaters. These locomotives had a maximum speed limit of and were designed to be able to haul at . A total of 282 were built. The PLM had prepared designs for another much larger locomotive by 1913, but it did not materialise as a result of the outbreak of the First World War.
thumb|Early 240P class 4-8-0
The second locomotive to appear in France was the famous 240P class of the Société Nationale des Chemins de Fer Français (SNCF), with "240" in this instance referring to the French classification of wheel arrangement according to the number and arrangement of axles rather than wheels. Technically, these locomotives were developments of some of the earliest Pacific locomotives in Europe that were built for the Chemin de Fer de Paris à Orléans (PO). The 240P class was considered to be one of André Chapelon's finest designs and benefited from his thorough understanding of thermodynamics and his appreciation of the need to consider the entirety of the steam circuit. The locomotive was a four-cylinder compound, fitted with Lentz-Dabeg poppet valves.
Coupled with an elegant French style tender, the second batch of the 240P class was aesthetically much more pleasing. With a power output of , the 240P class was reputed to have the highest power-to-weight ratio of any steam locomotive. Discussion continues as to how robust they were mechanically, for example whether the size of the bearings was too near the bone, or whether they were simply worked to death during the difficult war years. None have survived in preservation.
Hungary
thumb|[[MÁV Class 424]]
In the 1920s, the Magyar Államvasutak (Hungarian State Railways or MÁV) adopted the type as their standard mixed-traffic locomotive in the shape of the 424 class, which was built between 1924 and 1958. Of these, 365 were built for Hungary and 149 for foreign systems, including the Yugoslav Railways, the Soviet Union and North Korea. In Yugoslavia, they were designated the Jugoslovenske Železnice (JŽ) class 11. The last of the Hungarian locomotives were withdrawn from service in 1984.
Ireland
For a short period, the Great Southern and Western Railway in Ireland and its successor, the Great Southern Railways, operated two inside-cylinder shunting locomotives. Intended for shunting at Dublin Kingsbridge (now Heuston) and for banking on the steep gradient out of Cork Glanmire Road (now Kent), the first locomotive emerged from the company's Inchicore workshops in 1915 and the second in 1924. They proved unsuccessful, being expensive to operate and unsuitable for sharp curves, and were withdrawn in 1928 and 1931 respectively.
On the Irish three-foot gauge, the Londonderry and Lough Swilly Railway operated two tender locomotives. Built by Hudswell Clarke, they were introduced in 1905 and withdrawn in the 1930s and 1940s. The two were the only tender locomotives ever to operate on that gauge in Ireland and, with two subsequent tank engines from the same manufacturer, were considered to be the most powerful which ever worked on any Irish narrow-gauge railway.. It caused that majority of locomotives during WWII have been captured by USSR. After 1945 only 15 units returned to Poland. Their service ended in until 1970. Today the only surviving Os24 is stored in Stacja Muzeum in Warsaw. Locomotive was built in 1926 in Fablok.
Rhodesia
thumb|left|[[South African Class 7D 4-8-0|Rhodesia Railways 7th Class]]
Between 1899 and 1903, 52 Cape 7th Class 4-8-0 locomotives were built for the Beira and Mashonaland and Rhodesia Railways (BMR) that later became a constituent part of the Rhodesia Railways (RR). These locomotives were acquired by Southern Rhodesia at the time when railways were still expanding from the Cape Colony via Bechuanaland Protectorate into Southern Rhodesia in the southwest, from Beira in Mozambique to Umtali in the east, and while the Second Boer War was still in progress. At the time, the system was composed of four smaller railways, all of them still largely under construction, that were eventually all linked up in 1902. These were the fledgling Bechuanaland Railways (BR), the Mashonaland Railways (MR), the Rhodesia Railways Northern Extensions (RRM) and the BMR. The locomotives were ordered in five batches from three British manufacturers, 24 from Neilson, Reid and Company in 1899 and 1900, eight from Kitson and Company between 1901 and 1903, and twenty from North British Locomotive Company in 1903. In 1915, five of the Neilson, Reid locomotives were sold to the South African Railways, where they were designated Class 7D.
- Between 1899 and 1904, the New Cape Central Railway (NCCR) placed seven Cape 7th Class locomotives in service on its line from Worcester to Mosselbaai in the Cape Colony, built by Neilson, Reid and North British Locomotive Company (NBL). In 1925, when the NCCR was amalgamated into the SAR, these seven locomotives were designated Class 7E.
- Altogether 23 8th Class locomotives entered service on the CGR in 1902 and 1903, built by Neilson, Reid. They had larger coupled wheels than the 7th Class, bar frames, used saturated steam and had Stephenson valve gear. In service it was found that the four wheeled bogies and the slightly shorter fixed wheelbase made them steadier and easier riding than their predecessors. In 1912, they were designated Class 8 on the SAR.
thumb|[[South African Class 1 4-8-0|NGR Class Hendrie B, SAR Class 1]]
- Fifty Class B locomotives were delivered from NBL in 1904, the first tender locomotive fleet to be placed in service by the NGR. They had plate frames, Walschaert valve gear with Murdoch's D slide valves, Belpaire fireboxes and used saturated steam. Six of them were modified to a 4-8-2| Mountain wheel arrangement in 1906 and, in 1912, the remaining 44 were designated Class 1 on the SAR.
- In 1921, the London and South Western Railway (LSWR) built four G16 class two-cylinder tank locomotives to Urie's design, for use in the Feltham marshalling yard.
The London, Midland and Scottish Railway and the Southern Railway both contemplated the construction of tender freight locomotives, but these never materialised.
United States
In the United States, the was essentially a freight locomotive that was a development of the Consolidation. Most American locomotives were built in the late 19th or early 20th century. The type never achieved great popularity, although there were five occasions when a locomotive was considered as the heaviest and/or most powerful in the world upon its introduction. Those locomotives were the no. 20 Champion of the Lehigh Valley Railroad in 1880, the no. 229 Mastodon of the Central Pacific Railroad in 1882, the G5 class of the Great Northern Railway in 1897 and the no. 640 of the Illinois Central Railroad in 1899. It is noteworthy that the Great Northern G5 had piston valves, as large as the pistons of many locomotives then in service.
thumb|left|[[N&W 475|N&W class M no. 475]]
Even though, at the time, the wide-firebox Mikado had much more potential as far as speed is concerned, the Norfolk and Western Railway opted for the class M for its shorter wheelbase that enabled it to have over 90 percent of the locomotive's weight on the driving wheels, and the four-wheel leading truck for greater stability. The N&W operated from the early 1900s to the late 1950s. Built by Baldwin Locomotive Works from 1906 and nicknamed Mollies, the class M, class M1 and class M2 became the most numerous American class of .
thumb|[[Norfolk and Western Railway|Norfolk & Western class M2]]
The class M2 locomotives are often mistakenly believed to be the largest conventional built, but the Mexican PR-8 was over four tons heavier. Many of them lasted into the 1950s, but were poor steamers since the boiler's heating surface had been significantly increased compared to the classes M and M1, but with no corresponding improvement of firebox volume and grate area.
A total of six 4-8-0s are left to survive in the United States, Southern Pacific 2914, Norfolk and Western 433, 475, 1118, 1134, and 1151, and one of them, No. 475, continues to operate for the Strasburg Rail Road in Strasburg, Pennsylvania.
Zambia
In November 1953, the Zambezi Saw Mills Company (ZSM) in Northern Rhodesia (Zambia since 1964) purchased two Class 17 locomotives from the South African Railways for use on their Livingstone-Mulobezi logging railway. The company worked the teak forests that stretched to the north-west of Livingstone and it built one of the longest logging railways in the world to serve its sawmill at Mulobezi. The locomotives were scrapped between 1961 and 1963.