 All but two were built with a Franklin C2 type Booster engine on the trailing truck axle, following a successful trial of a booster on the smaller N class light lines 2-8-2.The booster allowed an additional tractive effort at starting and low speeds to increase the hauling power of the locomotive, particularly on heavy grades.X36 and X37 were built without boosters and were rostered for 'plains working' on the relatively flat line between Gheringhap and Maroona stations.