[2] The Q class's design stems from the requirement for a locomotive similar to the UB class with the inclusion of a wide Wootten firebox to burn poor quality lignite coal from the South Island and the Waikato.

Originally plans to equip the new locomotives with a Wootten firebox would have seen the "Camelback" configuration adopted, an arrangement which A. L. Beattie thoroughly rejected.

In operation, the locomotives proved to be satisfactory rather than brilliant and they suffered from occasional valve gear problems.

An improved slightly larger 'Q' type was ordered from Baldwins in 1914, but classified AA due to their dimensions similar to the A class.

[3] They saw out their final years working in Otago and the West Coast and the last Q class locomotive was retired in 1957.