In 1913, the maximum speed limit on the NIMT was raised to , reducing the journey time by 1 hour 25 minutes Auckland-Wellington or to 17 hours and between 30 and 45 minutes. Under T. Ronayne, the Railways Department general manager from 1895 to 1913, the section south to Parnell was duplicated and improvements made to the worst gradients and tight curves between Auckland and Mercer. Under his successor E. H. Hiley the second Parnell Tunnel with two tracks and an easier gradient was completed in 1915–1916. On the Kakariki bank between Halcombe and Marton a deviation reduced the 1 in 53 grade to 1 in 70 in 1915. Similar work was done to ease the gradient to Greatford, on the other side of the Rangitīkei River, in 1939. A 1914 Act authorised spending on the Westfield Deviation, new stations at Auckland and Wellington, track doubling (Penrose-Papakura, Ohinewai-Huntly, Horotiu-Frankton, Newmarket-New Lynn), and grade easements from Penrose to Te Kuiti, but the war delayed most of these works for over a decade.