The NZR W<sup>W</sup> class was a class of 4-6-4T tank locomotives that operated on the New Zealand national railway network. They were built for New Zealand Railways Department (NZR), and were the final development of the six-coupled tank engine in New Zealand, the penultimate class of tank locomotives to be built for NZR, and the first class of tank locomotives to be built with superheaters.
The W<sup>W</sup> class were based on the earlier W<sup>G</sup> class 4-6-4T tank locomotives, but with a reduced boiler pressure of 180psi and larger cylinders. The initial batch comprising locomotives W<sup>W</sup> 556-575 were built at Hillside Workshops in 1913, and initially carried the W<sup>G</sup> classification before being reclassified as the W<sup>W</sup> class, the 'W' indicating that these locomotives were intended for suburban work in Wellington.
Following the success of the first 20 locomotives, Hillside delivered a further 30 locomotives, which were classified W<sup>W</sup> from new. A further 14 locomotives were rebuilt from the W<sup>G</sup> class at Hutt Workshops in two batches between 1940âÂÂ42 and 1950-52.
Although most of the class were built with piston valves, W<sup>W</sup> 565 was delivered with slide valves, which were not suited to superheated steam, leading to their replacement with standard piston valves.
The W<sup>W</sup> class demonstrated to be extremely versatile in service, being capable of handling almost any task. Initially both Auckland and Wellington received allocations of these locomotives which were used in suburban passenger service and occasionally on goods work; later allocations saw engines of this class allocated to the West Coast at Westport and Greymouth. Here, the locomotives worked mostly on coal trains and occasionally hauled branch line passenger services.
During the early 1950s, the decision was made to fit new higher-pitched boilers with deeper fireboxes to four W<sup>G</sup> class locomotives, Nos. 479, 480, 486 and 488, as a trial when they were rebuilt to W<sup>W</sup> class specifications. This required a new type of smokebox saddle, cab and enlarged coal bunker. This alteration was deemed to be successful, adding an extra 100 sq. ft. of heating surface; rather than rebuild the remaining six W<sup>G</sup>s due to the poor condition of their frames, NZR decided to rebuild 12 existing W<sup>W</sup> class locomotives, Nos. 571, 573-575, 644, 669, 672, 678-680, 683-684.
During the early 1960s, several of the W<sup>W</sup> class locomotives transferred from the North Island to Greymouth on the West Coast were altered to operate on the Rewanui Incline by the provision of 'trap-door' cowcatchers to clear the centre rail and additional air reservoirs on the tops of the side tanks. Following the removal of the centre rail in 1966, the locomotives' extra reservoirs were removed and the original brake pump was replaced by two larger pumps, one on each side of the smokebox.
The first member of the W<sup>W</sup> class to be withdrawn was W<sup>W</sup> 491, which was withdrawn from service in August 1955 for use as a sectionalised aid for apprentice training at Otahuhu Workshops. A further 43 locomotives were withdrawn by the first half of the 1960s, by which time the remaining 21 locomotives were in service in Auckland and on the West Coast. Further withdrawals were at a slower pace, with a final seven locomotives still in service on the West Coast at the beginning of 1969.
Four W<sup>W</sup> class locomotives have been preserved, all in the North Island of New Zealand. Three were rebuilt with higher-pitched boilers while the fourth retains its original low-pitched boiler:
In addition, Steam Incorporated also owned an original low-pitched W<sup>W</sup> boiler which had originally been fitted to W<sup>W</sup> 646. This boiler was purchased for industrial use at the Ford Motor Company plant in the Hutt Valley in the 1970s and was stored at Paekakariki until early 2015 when it was sold to MOTAT to aid the future restoration of W<sup>W</sup> 491.