The HAA wagons were introduced in response to the increasing demand for electricity in the 1960s. Several new generation power stations were built which required a continuous supply of coal at a rapid rate. A new design of wagon was developed, the HAA, that could be loaded and then emptied via underfloor doors, while on the move in what were termed ‘Merry-go-Round’ (MGR) trains. 

Following the construction of two protypes at Darlington Works in 1964/1965, a staggering 10,702 hopper wagons were built at Shildon Works, with a further 160 turned out at Ashford. Another 460 of the ‘HDA’ variant were built at Shildon, these being basically the same, but had improved braking to allow them to run at 60mph rather than 45mph. 

All versions of the HAA carried 33-tonnes of pulverised coal. Other variants of the HAA included HBA, HCA, HFA, HMA and HNA all of which had subtle differences but were essentially similar. The last MGR trains ran to Longannet power station in Scotland during 2008 and after that, only a small number of a CDA variant, converted for use on China clay traffic, remained in use.

The original and most common of the MGR hopper wagons, these are presented in 1980s BR Railfreight livery with red frames. Each of the wagons has a different running number.