Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a 2-8-2 is a locomotive with a two-wheel leading truck, eight driving wheels (four driven axles), and a two-wheel trailing truck. The first 2-8-2 locomotives ever built were some 3'6" gauge locomotives for Japan constructed by Baldwin in 1898, and they were dubbed the Mikado type by Baldwin employees, presumably in reference to Gilbert and Sullivan's 1885 musical The Mikado. The name stuck, especially in the United States, although during World War II attempts were made by some to change the name to MacArthur, but to no avail. It was sometimes contracted to "Mike"
Other 2-8-2 types were produced in small numbers after that, notably compound locomotives for the Santa Fe, but the first large order was by the Northern Pacific in 1904. The railroad desired a mountain-climbing locomotive to haul the trains that on level track required a 2-6-2 "Prairie" type. The 2-8-2 fulfilled their needs, and the NP bought 386 of the type.
In essence, the 2-8-2 is a locomotive with the adhesion and tractive effort of a 2-8-0 "Consolidation" type but capable of greater power and speed, made possible by the larger firebox and boiler possible with a trailing truck. The firebox is no longer limited by having to fit between or above the driving wheels of the Consolidation but instead fits behind them, where it can be both long and wide.
Another way to look at the 2-8-2 is that it is the freight version of the 4-6-2 "Pacific" type. With its smaller driving wheels, the 2-8-2 and 4-6-2 were approximately the same length, and many railroads produced a pair of locomotive types utilising similar boilers on different running gear. The Pennsylvania Railroad, indeed, produced both 574 of the L1s Mikado and 425 of the K4s Pacific using the exact same boiler and many other common components including the cab and trailing truck - 999 locomotives with the same boiler.
The 2-8-2 replaced the 2-8-0 as the premier freight hauler on America's railroads, and over 14,000 of this type were eventually built worldwide; one out of five US locomotives was a 2-8-2 by the 1920s. 2-8-2s were chosen as one of the USRA Standard locomotive types, and the USRA produced both a light and heavy Mikado type, the light being for railroads whose physical plant could not tolerate the extra axle loading of the heavy type. The USRA heavy Mikado was very close to the Pennsy's L1s in most respects, except for having a round-topped firebox instead of the PRR's trademark Belpaire firebox. 625 of the USRA light Mikado and 233 of the USRA heavy Mikado were produced during the USRA's control of the American railroads; after it was disbanded, many railroads ordered further copies of the same basic design, though sometimes customised in various ways.
Railroads that used the 2-8-2 in a big way included the New York Central with 724, the B&O with 664 examples, the Santa Fe with 308, the Canadian Pacific with 334, and most other railroads. Notable exceptions to the rule included the Norfolk and Western, who had none.
With the introduction of heavier trains and faster schedules, the Mikado started to have trouble keeping up. 4-8-2 'Mountain' types had always hauled some of the faster express freight, but the introduction of the Superpower Steam concept by Lima and others was what really challenged the type. Locomotives with a four-wheel trailing truck and therefore much more available steam production for power, such as the 2-8-4 'Berkshire' and the 4-8-4 began to make heavy inroads. Nowhere was the 2-8-2 completely supplanted, but it lost to top-flight stuff on most railroads.
The last standard gauge Mikados produced in North America were in 1948 for the Canadian Pacific; some narrow gauge examples for the Newfoundland Railway followed in 1949, and these were the last built on the continent.
Most American railroads ran Mikados till the end of steam in the 1950s and 1960s.
In England, the 2-8-2 found little favor. Sir Nigel Gresley of the LNER designed two types. His P1 was a handsome Mikado freight locomotive, the freight version of his A1 Pacifics (think "Flying Scotsman") - but it was simply too much locomotive for the short and slow unfitted (unbraked) freight trains run in England at the time. His subsequent P2 class was a beautifully proportioned, streamlined express passenger locomotive for Scotland, intended to haul heavy expresses from Edinburgh to Aberdeen. Lack of self-centering on the leading truck and insufficient lateral motion on the leading pair of driving wheels meant they were rough on the track, and Gresley's successor Edward Thompson rebuilt them as rather ugly and conventional Pacifics.
There were a somewhat larger number of 2-8-2 tank locomotives, however.
In Europe, the type was very popular in France, especially for mixed traffic locomotives, and in Germany for fast freight locomotives (class BR 41) and the Prussian BR 39 express locomotive for mountain territory.