Editing 4-4-4-4 (section)

==History==
===20th century===
[[Image:Baltimore and Ohio Duplex detail.jpg|thumb|Rearward cylinders and gear of the sole N-1]]
[[Image:Pennsylvania Railroad TR1.JPG|thumb|The [[Pennsylvania Railroad]]'s controversial [[Pennsylvania Railroad class T1|class T1]] duplex locomotive]]
The first locomotive built with this arrangement was the [[Baltimore and Ohio Railroad]]'s sole [[Baltimore and Ohio Class N-1|class N-1 #5600 ''George H. Emerson'']], constructed at the B&O's own [[Mount Clare Shops]] in May 1937. To reduce the fixed wheelbase, this locomotive had the two sets of cylinders at opposite ends, so that the rear pair were beside the firebox. The firebox was an unconventional water tube design which was tested well on other locomotives and needed a large space. The 4-4-4-4 wheel arrangement proved to be a poor design, as it restricted the firebox size and exposed the cylinders to dust and dirt, causing premature wear. The locomotive was not considered successful enough to duplicate. The sole N-1 was retired in 1943 and scrapped in 1950.<ref>{{cite book |last=Reed |first=Brian |date = June 1972 |series = Loco Profile 24 |title=Pennsylvania Duplexii |location=Windsor, Berkshire |publisher = Profile Publications Limited |pages=266–267 }}</ref>

The next locomotives to use the 4-4-4-4 arrangement were the [[Pennsylvania Railroad]]'s 52 [[Pennsylvania Railroad class T1|class T1]] locomotives (although the [[Pennsylvania Railroad class S1|single class S1]] was originally intended to be a 4-4-4-4, it was later changed to 6-4-4-6 due to increased weight). These had the cylinders in front of the wheels they drove, so that the rear pair were between the two sets of drivers. These locomotives were impressive performers but suffered from wheelslip and severe reliability problems, and did not last long in service. {{sfn|Staufer|1962|p=217}} 

The two prototypes, #6110 and #6111, were constructed at the [[Baldwin Locomotive Works]] in April 1942. The mechanical design work had been undertaken by Baldwin's chief engineer Ralph P. Johnson, while the streamlining was the handiwork of industrial designer [[Raymond Loewy]].{{sfn|Staufer|1962|p=217}}{{sfn|Staufer|1962|p=225}} Both locomotives showed fantastic performance in test trials, leading to the construction of 50 further examples. Numbers 5500 through 5524 would be constructed at the PRR's [[Altoona Works]] while 5525 through 5549 would be constructed at Baldwin. 

As operation continued, problems arose. The T1 class was known for violent uncontrollable wheel slip as well as performance and maintenance issues caused by the [[Franklin Railway Supply|Franklin]] oscillating-cam [[Poppet valve]]s.{{sfn|Staufer|1962|p=218}} The PRR did show interest in trying to solve these problems such as replacing the original poppets on #5500 with easier to maintain Franklin rotary-cam poppet valves and refitting #5547 with [[Walschaerts valve gear]]s reclassifying the latter engine as a "T1a".{{sfn|Staufer|1962|p=218}}

The PRR decided to stop development of the T1 class, replacing all examples by the mid-1950s with diesel units. All members of the class were scrapped. It is very likely the T1 may have had many of its shortcomings solved had the PRR not retired the short-lived class so soon.{{sfn|Staufer|1962|p=218}} During its short service life, the T1 was rumored to have operated at higher speeds than the steam speed record set by the [[London and North Eastern Railway]]'s [[LNER Class A4 4468 Mallard|''Mallard'']], but the claims could not be officially verified.<ref>{{Cite web |url=http://www.nrm.org.uk/~/media/files/nrm/pdf/archiveslists/tech-files/1975-7007-mallard.pdf |archive-url=https://web.archive.org/web/20171031101021/http://www.nrm.org.uk/~/media/files/nrm/pdf/archiveslists/tech-files/1975-7007-mallard.pdf |url-status=live |archive-date=31 October 2017 |title=Railway Vehicle Information Sheet - 1975-7007 - Mallard |website=National Railway Museum Official Website |publisher=National Railway Museum |location=York England |page=3 |language=English |type=PDF File |access-date=9 May 2017 }}</ref>

Several bush tramway engine types were built with this wheel arrangement. These were known as "16-Wheelers". Two notable [[New Zealand]]-based builders were J. Johnson and Sons Ltd. in [[Invercargill]] and [[A & G Price]] in [[Thames, New Zealand|Thames]].

===21st century===
Only one locomotive of the 4-4-4-4 arrangement is being constructed. When finished, [[Pennsylvania Railroad 5550|PRR 5550]] will be the 53rd and only surviving example of the PRR T1 class rather than a replica. The new build T1 is expected to implement some design improvements to help aid in solving problems encountered by the original 52. The estimated date of project completion is currently set to be around 2030.<ref name="T1Trust01">{{Cite web |url=https://prrt1steamlocomotivetrust.org/faq.php |title=FAQ Section - The T1 Trust |date=2016 |publisher=The Pennsylvania Railroad T1 Steam Locomotive Trust |access-date=23 April 2017 |archive-date=24 August 2019 |archive-url=https://web.archive.org/web/20190824010545/https://www.prrt1steamlocomotivetrust.org/faq.php |url-status=dead }}</ref> As of November 2023, the project is 43.3% complete with an estimated total cost of US $7,000,000.<ref name="T1Trust02">{{Cite web |url=https://prrt1steamlocomotivetrust.org/station/index.php?route=tinfo/tinfo |title=5550 Tender Preservation Fund |date=2017-08-21 |website=prrt1steamlocomotivetrust.org |publisher=Pennsylvania Railroad T1 Steam Locomotive Trust |location=Pottstown, Pennsylvania, USA |language=en |publication-date=August 2017 |access-date=2017-08-21 |archive-date=2017-08-21 |archive-url=https://web.archive.org/web/20170821170036/https://prrt1steamlocomotivetrust.org/station/index.php?route=tinfo/tinfo |url-status=dead }}</ref>