The HAA Coal Hoppers were designed for use as part of the new Merry-Go-Round (MGR) trains introduced in the 1960s. Replacing the ageing fleet of wooden- and steel-bodied wagons, like the 16 Ton Minerals, the HAAs were air braked, had double the capacity of many of their predecessors, and could operate more efficiently thanks to the MGR system which allowed for on-the-move loading and unloading where facilities existed.