New Zealand is a country in which it has not been easy to build railways. It is a land of mountains and high hills, deeply scarred by river valleys, with an upthrust coastline that in many areas is notoriously unstable for civil engineering works. Even on the relatively few open plains, such as are found in Canterbury, railway builders have had to contend with swift-flowing, unpredictable rivers. More than 2,600 bridges and viaducts carry railway tracks over river and gorge. Some of these bridges are noteworthy for their length; others are relatively short but high. The longest railway bridges are in the South Island, over the sprawling, snow-fed rivers that cross the Canterbury Plains on their way from the Southern Alps to the sea. The Rakaia River bridge, for example, is over a mile in length. By comparison, the longest bridge in the North Island – across the south end of the Tauranga Harbour – is 1,400 ft long. On the other hand, most of the very high steel viaducts are located in the North Island, although there are some of considerable height on the Midland and Central Otago lines in the South Island. Highest of all New Zealand railway viaducts is the one over the Mohaka Gorge, on the line between Napier and Gisborne. It stands 312 ft above the river. On the same railway are four other viaducts ranging in height from 215 ft to 255 ft. In the mountainous central region of the North Island, the Main Trunk line crosses several spectacular steel viaducts, of which Makatote (258 ft) and Makohine (238 ft) are the highest, and Mangaweka (946 ft) the longest. The most outstanding of the South Island viaducts is the one at Staircase, on the Midland line. Spanning a tributary of the Waimakariri at a point where it joins the main river, the viaduct is 235 ft high.

Although the early railway builders did their best to reduce construction costs by avoiding tunnels wherever possible, they frequently found that tunnelling was cheaper than going round an obstruction. Even so, most of the tunnels driven during the nineteenth century were of very restricted profile, being just large enough to accommodate the small locomotives and rolling stock of the time. This legacy from the days of the “Vogel Railways” has seriously handicapped New Zealand railway operations, in that until recent years it was not possible to construct locomotives and rolling stock higher than 11 ft 6 in. above rail, or wider than 8 ft 6 in. In 1955 a start was made with the lowering of the track in certain small-bore tunnels on the North Island Main Trunk line, thus permitting the operation of American-built diesel locomotives with a maximum height of 12 ft 2 in. and a width of 9 ft 2 in.

At 31 March 1965 there were no fewer than 189 tunnels on the railway system, with an aggregate length of about 54 miles. Six of these tunnels are more than a mile long, and two, Otira and Rimutaka, are 5 ¼ and 5 miles long respectively. The Otira Tunnel, which pierces the Southern Alps and serves as a gateway between Canterbury and Westland, was started in 1908 but was not officially opened for traffic until 1923. Modern machinery and techniques enabled the slightly longer Rimutaka Tunnel to be built in much quicker time, the main contract taking little more than four years to complete. The Rimutaka Tunnel and its associated works, consisting of 14 miles of new line and one short tunnel, was probably the greatest railway improvement project ever undertaken in New Zealand. When completed, in 1955, it eliminated the notorious Rimutaka Incline, with its 1 in 15 grade, and costly, time-consuming haulage by special “Fell” locomotives and brake vans.

An even more ambitious project, necessitating 15 ½ miles of new railway and a 5 ½-mile tunnel, is now being undertaken to link the eastern Waikato district with the Bay of Plenty. Known as the Kaimai deviation, the new line will replace 35 miles of old railway in need of extensive and costly upgrading. The reduced haulage distances offered by the new route will not only enable valuable savings to be made in railway operating costs, but will also be of benefit to industry in that freight charges on goods to and from the Bay of Plenty will be reduced.

The highest elevation reached by a railway in New Zealand is 2,671 ft near Pokaka, on the North Island Main Trunk line. At four other points on the railway system, including two in the South Island, altitudes of more than 2,000 ft are reached. Gradients of 1 in 50 to 1 in 80 are common on main lines, and in a few localities on old main and branch lines grades as steep as 1 in 35 to 1 in 40 will be found. More than one-third of the 423-mile length of the Wellington-Auckland main line is graded at 1 in 100 or steeper.

The steepest gradients on main lines are 1 in 33 from west to east through the electrified Otira Tunnel; 1 in 35 on the Taranaki line north of Wanganui; and 1 in 35 on the Rotorua line where it crosses the Mamaku Ranges. The longest continuous gradient is between Rolleston and Springfield, on the Midland line, where the average inclination of 1 in 147 is maintained for 30 miles.

Not only gradients are responsible for railway operating difficulties in New Zealand. Curvature also is severe, this being the result both of topography and of financial considerations. Curves of 7 ½ chains radius are frequent on some of the main lines, and on some older routes the gradients are not compensated for curvature. Really long sections of dead-straight track are rare, and even on the Canterbury Plains the longest is only 15 ½ miles.