Belgian Railway type 12bis

In order to be able to burn cheap coal with the smallest fragments, the Belgian State Railways needed a large firebox for their express locomotives. This also applied to the type 12bis, which was based on the type 12 from 1888 and where the “bis” stood for “second”. In order to achieve the required power, the grate area had to be further increased to 4.7 square meters or 50 square feet. 

On the 12bis, too, the firebox consisted of a narrow, front part between the wheels of the second driving axle and a wide, rear part above the trailing axle. It still had outside frames, as well as serve tubes and a conical chimney. It was said to run smoothly even at 110 km/h (68 mph).