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 (Originally they were to be classed QB but as they were to supplement the A class, given their aforementioned designation.) 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 World 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.
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 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. 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.