Belpaire's purpose behind his invention was to create a firebox that was easier to manufacture by utilizing fewer staybolts and simplfying the overall design. However, in doing so this resulted in the squarish contraption being somewhat harder to attach to a circular boiler. Additionally, the flat design caused cleaning to be a much more difficult process. Still, it also had other advantages over standard designs of the day that were concave and molded roughly to the shape of boiler; due to its greater surface area at the top (also known as the crown sheet) the Belpaire offered better heat transfer and increased steam production. In general, the firebox was more efficient. Aside from its operational advantages the design is also said to have been less stressful on the staybolts, which attached the steel plates of the firebox at right angles instead of at perpendicular angles on concave plates.
In any event, the Pennsylvania was unconcerned with any potential setbacks the Belpaire Firebox may have possessed. The railroad's famous Altoona Shops in the Keystone State outshopped virtually all of its home-built steam locomotives after 1885 with its own version of the Belpaire. Wheel arrangements of the Pennsy that featured the firebox included 0-4-0s, 2-6-0 Moguls, 2-8-2 Mikados, 2-10-2 Sante Fes, 2-10-0 Decapods, 4-4-2 Atlantics, 2-8-8-0 Consolidation Mallets, and of course the classic 4-6-2 Pacifics among others. The Great Northern was the only other road to utilize the Belpaire (which designed its own custom version like the Pennsy) in vast numbers operating them on 2-6-0s, 2-8-2s, 4-6-2s, 2-10-2s, 4-8-2 Mountains, 4-8-4 Northerns, 2-6-6-2s, the unique Class M-2 2-6-8-0, and others.
Notable other roads (or their predecessors) that fitted the Belpaire to at least one of their steamers included the Burlington (0-10-0s, 4-6-0, 0-6-2s, 2-6-0s, 2-6-2s and 2-6-6-2s), Soo Line (4-6-0s, 2-8-0s), Chesapeake & Ohio (4-4-0s, 2-8-0s), Union Pacific (4-8-0 Mastadon/Twelve-Wheelers), Louisville & Nashville (2-6-0s), Toledo Peoria & Western (2-8-0s), Pere Marquette (4-4-2s, 2-6-0s, 2-8-0s), Spokane Portland & Seattle (4-4-2s, 4-6-2s), Grand Trunk Western (4-6-2s), Southern (2-6-0s), Minneapolis & St. Louis (2-8-0s), Illinois Central (2-8-0s, 4-8-0s), Pittsburgh & West Virginia (2-6-6-2s), Rock Island (4-4-2s), Richmond Fredericksburg & Potomac (2-6-0), Buffalo Rochester & Pittsburgh (4-8-0s), Monon (4-8-0s), and the New York Central (4-6-0).
Other Western Hemisphere railroads to use the design outside of the United States included the
Canadian Pacific, Cuban Central, Mexico Cuernavaca & Pacific,
Temiskaming & Northern Ontario, and the Mexican Railway. Most
applications of the Belpaire could be found in Europe where it was
originally invented and saw widespread use on British lines such as the
Great Western Railway and London, Midland & Scottish Railway.
Additionally, Australia's Victorian Railways fitted many of its steamers
(Thanks to Peter Getz for help with the information and history regarding the Belpaire Firebox.)
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