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 small fragments of coal, 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.