Built to a similar but vastly improved design to the Q class of 1901, they had a wheel arrangement of 4-6-2 and were suited to hauling freight services in the North Island The United States built locomotives were an urgent order needed due to heavy demand with both New Zealand and British workshops unable to supply as the First World War was draining resources and manpower for the war effort. Construction was completed less than two months after the order was placed and all ten entered service in New Zealand in June 1915. Bar frames were used by the American builders instead of NZR's preferred plate frames, and the class had superheaters as built. Water capacity was double that of the A class and coal a quarter more.
Service
They were worked hard for four decades being supplanted by the more powerful "K" and "J" class families. In 1919 No. 654 was used for a very short experiment in the use of pulverized coal. Only one trip is known to have been made before the engine was converted back. They were initially equipped with grates smaller than either the A or AB class. Not an issue with hard coal, increasing use of soft Waikato coal meant a larger grate was desirable. Thus the entire class had their boilers replaced with AB class types starting in 1940 giving the reliable locomotives more life. This soon became a blessing when they were worked hard through the Second World War. They were used extensively on the steeply graded central NIMT, Wanganui and Gisborne sections. In the mid-1950s most of the class were still in reasonable condition and the engines were regarded as a very successful class, capable of taking somewhat heavier loads than an Ab. Serious consideration was given to a major refit of the class rather than rebuilding 9 of the mid 1930s K class which were essentially worn out, and very expensive to run and repair, after being run hard and fast to cope with heavier freight traffic in the early 1950s and the attempts of enthusiastic drivers to outrun the new diesel competition. The option of modernising the Aa class was rejected on account of their age, and due to the fact the cultural importance of K's made them impossible to withdraw in 1956. The first withdrawals occurred in December 1955, when six were taken out of service with their boilers returned to the AB class boiler repair pool. The remaining four continued to operate until February 1957. None are preserved.
