The Belpaire firebox is a type of firebox used on steam locomotives. It was invented by Alfred Belpaire of Belgium. It has a greater surface area at the top of the firebox, improving heat transfer and steam production. Its rectangular shape makes attaching the firebox to the boiler more difficult, but this is offset by simpler interior bracing of the firebox.
The Pennsylvania Railroad used Belpaire fireboxes on nearly all of its steam locomotives. The distinctive square shape practically became a PRR trademark, as no other American railroad except the Great Northern used Belpaire fireboxes in significant numbers.
In Britain, the Belpaire design was a standard feature on most Great Western Railway locomotives, and a significant number of the London, Midland and Scottish Railway's locomotives also employed the design. Some other British railway companies used the Belpaire firebox on a handful of locomotives, but not to any major extent.