The type 6 was an express locomotive designed for long inclines on the line between Brussels and Luxembourg. The requirements included that it should be able to haul a train of 100 tonnes over long inclines of 1.6 percent with a speed of 65 km/h. Since it had to burn coal dust, the grate had to be extraordinarily large. This led to three prototypes with a very special boiler design and a total of 32 production locomotives which were more conventional. Typical for locomotives designed by Belpaire, they had outside frames and inside cylinders.

The first prototype was built by Cockerill in 1884. Since the boiler of the previous type 2 0-6-0 was not big enough, the type 6 received a leading axle that was mounted in a radially adjustable Adams axlebox. The wide and shallow firebox had a grate of 6.7 square metres (72 square feet) and four doors. The water spaces of the firebox and boiler barrel were not directly connected, there was only a connection between both domes. Like in American camelback locomotives, the driver had a small cab that was located in front of the firebox, but in this case only on the right side of the boiler.

The prototype was difficult to operate and maintain. Later it even got a second carrying axle between the second and third driving axle. Two more prototypes were built in a similar fashion, but without the additional carrying axle. Series production of a modified variant was started in 1888. This had a minimally smaller grate of only 5.74 square meters (62 square feet), but only one cab and a boiler with only one water space. The cylinders, pistons, valves, valve gear and crank axle were shared with the types 12 and 25. The chimney was of the rectangular type also seen on other Belgian locomotives and had a variable blast pipe. 

The three prototypes were withdrawn in 1900. Although the production locomotives delivered roughly 1,000 hp, the four firebox doors were difficult to feed and the maintenance of the firebox was complicated. So the type 6bis was created in 1904 by rebuilding eight type 6 locomotives with a new boiler. This had a much smaller grate and a higher pressure. When burning coal briquettes, it delivered more power than the original boiler fired with coal dust. Many non-rebuilt locomotives were scrapped between 1907 and 1911. After World War I, only one locomotive each of type 6 and 6bis were remaining, which were withdrawn in 1922 and 1923.