Editing 4-8-4 (section)

===Development===
[[File:Union Pacific 844, Painted Rocks, NV, 2009 (crop).jpg|thumb|left|[[Union Pacific 844]], the only steam locomotive never retired by a [[North America]]n [[Class I railroad]]]]
The {{nowrap|4-8-4}} wheel arrangement was a progression from the [[4-8-2|{{nowrap|4-8-2}} Mountain]] type and, like the [[2-8-4|{{nowrap|2-8-4}} Berkshire]] and [[4-6-4|{{nowrap|4-6-4}} Hudson]] types, an example of the "Super Power" concept in steam locomotive design that made use of the larger [[Firebox (steam engine)|firebox]] that could be supported by a four-wheel trailing truck, which allowed greater production of steam. The four-wheel leading truck gave stability at speed and the eight driving wheels gave greater adhesion.  The {{nowrap|4-8-4}} type evolved in the United States soon after the [[Lima Locomotive Works]] introduced the concept of "Lima Super Power" in 1925, making heavy [[2-8-2]] and [[2-8-4]] locomotives. The prototype was built by [[American Locomotive Company]] (ALCO) for the Northern Pacific Railway (NP) in 1926, with a very large firebox with a {{convert|100|sqft|m2|adj=on}} grate, designed to burn low quality [[lignite]] coal. The four-wheel trailing truck weighed about {{convert|15000|lb|t}} more than two-wheel trucks of the time and could carry an additional {{convert|55000|lb|t}} of engine weight; the difference of {{convert|40000|lb|t}} was available for increased boiler capacity.<ref name="Alfred W 1952">Alfred W. Bruce. (1952). ''The Steam Locomotive in America: Its Development in the Twentieth Century''. New York. pp. 296, 299, 308–309.</ref>

[[File:Timken 1111.jpg|thumb|350px|Roller-bearing equipped [[Timken 1111]] (Northern Pacific 2626)]]
The {{nowrap|4-8-4}} type arrived when nearly all the important steam locomotive design improvements had already been proven, including [[Superheater]]s, [[Fireman (steam engine)#Mechanical stoker|mechanical stoker]]s, outside [[valve gear]] and the Delta trailing truck. One-piece, cast steel bed-frames with integrally cast cylinders gave the strength and rigidity to use [[Rolling-element bearing#Roller bearings|Roller bearings]]. In 1930, the [[Timken Company]] used the [[Timken 1111]], a {{nowrap|4-8-4}} built by ALCO with roller bearings on all axles, to demonstrate the value of their sealed roller bearings. The Timken 1111 was subsequently sold to the NP, where it became NP No.&nbsp;2626, their sole Class A-1 locomotive.<ref name="Alfred W 1952"/>

The stability of the {{nowrap|4-8-4}} wheel arrangement meant that driving wheels up to {{convert|80|in|m}} diameter could be used for high speed passenger and fast freight operation. Lateral control devices allowed these locomotives to traverse relatively sharp curves despite their {{nowrap|eight-coupled}} drivers. The increased boiler size possible with this type, together with the high axle loads permitted on mainlines in North America, resulted in the design of some massive locomotives, some of which weighed as much as 450 tons, including the tender. The {{nowrap|4-8-4}} was suitable for both express passenger and fast freight service, though it was not suited to heavy [[drag freight]] trains.<ref name="Alfred W 1952"/>

Although locomotives of the {{nowrap|4-8-4}} wheel arrangement were used in a number of countries, those that were developed outside North America included various design features which set them apart from North American practice. Scaled down examples of the type were exported by two American builders, ALCO and [[Baldwin Locomotive Works]], for {{RailGauge|1000mm|allk=on}} lines in [[Brazil]]. Most were two-cylinder locomotives, but five classes of three-cylinder {{nowrap|4-8-4}}s were built:
* The simplex [[DRB Class 06|06 class]] by the [[Deutsche Reichsbahn]] in [[Germany]].
* The simplex [[Victorian Railways H class|H class]] by the [[Victorian Railways]] in [[Australia]].
* The compound 242A1 class of the Société nationale des chemins de fer français ([[SNCF]]) in [[France]].
* The simplex [[ČSD Class 477.0|477 class]] tank engines by [[ČKD|CKD]] for [[Czechoslovak State Railways]]; 60 4-8-4T locomotives were built between 1951 and 1955, the last new steam engines built in [[Czechoslovakia]]. 
* An experimental high-pressure compound locomotive of the [[New York Central]] (NYC).