In the first decade of the twentieth century the pattern of the national railway system became more clearly defined. Wellington had long been connected with New Plymouth and Napier, and Auckland with Rotorua and Thames. The North Island Main Trunk Railway, reaching into the remote and mountainous highlands of the King Country, was officially declared completed on 6 November 1908, when the northern and southern railheads met in the forest at a point almost midway between Auckland and Wellington. By far the greatest project of its kind ever attempted in this country, the 426-mile railway had taken more than 40 years to build.

Between the end of the First World War in 1918 and that of the Second World War in 1945, several other vitally important rail links were completed. Westland was connected with Canterbury in 1923 by means of the 5 ¼-mile Otira Tunnel; Whangarei was connected with the main North Island system in 1925, followed by Tauranga in 1928; New Plymouth gained a direct connection with Auckland in 1933; Gisborne and Dargaville were joined with other North Island lines in 1942; and the isolated Westport section became part of the South Island system in 1943. Two years later, in 1945, the Christchurch-Picton main line was completed, thus providing through rail communication between Queen Charlotte Sound and Bluff. The decade of the fifties saw the building of new railways to tap the vast exotic forests of the North Island pumice lands. An 18-mile branch line from Putaruru to Tokoroa and Kinleith was completed in 1952, and 50 miles of new line to serve the Tasman Pulp and Paper Co's. mills at Kawerau were brought into use between 1955 and 1957.

The trend in the immediate future would appear to be one of consolidation rather than expansion. Already many uneconomic lines have been closed. Thus New Zealand's railway route milage, which reached its peak of 3,558 miles in 1953, had been reduced to 3,254 miles by 1 April 1965. It could shrink even further before the next phase of construction begins.