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, however this is offset by simpler interior bracing of the firebox.
The Pennsylvania Railroad used Belpaire fireboxes on nearly all of their steam locomotives. The distinct square shape practically became a PRR trademark, as no other American railroad except the Great Northern used Belpaire fireboxes in significant numbers.