Editing 4-4-4-4 (section)

{{Short description|Duplex locomotive wheel arrangement}}
{{Infobox steam wheel arrangement
| name             = 4-4-4-4
| image            = WheelArrangement 4-4-4-4.svg
| alt              = Diagram of one small leading wheel, and four large driving wheels joined by a coupling rod
| caption          = Front of locomotive at left
| image2           = Image:Baltimore and Ohio Duplex.jpg
| alt2             = 
| caption2         = The [[Baltimore and Ohio Railroad]]'s [[Baltimore and Ohio class N-1|#5600]] ''George H. Emerson''.
<!--Equivalent classifications-->
| hatnote          = 
| UIC/Germany/Italy= 2'BB2'
| French/Spanish   = 2222
| Turkish          = 2424
| Swiss            = 4/8
| Russian          = 2-2-2-2
<!--First known tank engine version-->
| date             = 
| country          = 
| locomotive       = 
| railway          = 
| designer         = 
| builder          = 
| evolvedfrom      = 
| evolvedto        = 
| mainbenefit      = 
| maindrawback     = 
<!--First known tender engine version-->
| date2            = May 1937
| country2         = [[United States]]
| locomotive2      = [[Baltimore and Ohio Class N-1|B&O Class N-1]]
| railway2         = [[Baltimore and Ohio Railroad]]
| designer2        = Baltimore and Ohio Railroad
| builder2         = [[Mount Clare Shops]]
| evolvedfrom2     = 
| evolvedto2       = 
| mainbenefit2     = Decreased wheelbase
| maindrawback2    = Premature cylinder wear, cylinder clogging and restricted firebox size
<!--First known "True type" version-->
| date3            = April 1942
| country3         = United States
| locomotive3      = [[Pennsylvania Railroad class T1|PRR class T1]]
| railway3         = [[Pennsylvania Railroad]]
| designer3        = Ralph P. Johnson<br>[[Raymond Loewy]]
| builder3         = Baldwin Locomotive Works<br>[[Altoona Works]]
| evolvedfrom3     = [[4-8-4]]
| evolvedto3       = 
| mainbenefit3     = Better high speed performance
| maindrawback3    = [[Wheel slip]]
}}

A '''4-4-4-4''' [[steam locomotive]], in the [[Whyte notation]] for describing locomotive [[wheel arrangement]]s, has a four-wheel [[leading truck]], two sets of four [[driving wheel]]s, and a four-wheel [[trailing truck]]. While it would be possible to make an [[articulated locomotive]] of this arrangement, the only 4-4-4-4s ever built were [[duplex locomotive]]s&mdash;with two sets of cylinders driving two sets of driven wheels in one rigid frame, essentially a [[4-8-4]] with [[divided drive (locomotive)|divided drive]].