In 1901/2 two Q class bogie bolster wagons (4473, 4494) were altered to carry locomotive boilers. Initially they had a single pivoting central bolster. Later two more were added directly over the bogie centre-lines.

In 1931 they were classified separately as class QB. In 1963 QB4473 written off and Q4709 was altered to QB standard to replace it. The demise of the steam locomotive rendered these wagons redundant and in 1967 number 4709 was converted to class QBB whilst 4494 was written off in 1974.