 In 1903, chief draftsman G. A. Pearson designed what was to become the eventual WF class tank locomotive.The new locomotive proposed was a modestly-sized 2-6-4T tank locomotive with 45-inch driving wheels and an adhesive weight of just under 27 tons.The Walschaerts valve gear actuated inside-admission piston valves, which would later become a standard on all subsequent NZR designs.