It has been said that Raurimu Spiral would never have been constructed if aeroplanes had existed when the reconnaissance surveys were made. When they actually did fly over in later years a lesson in aviation was written in the spiral for them to read. In the words of James Cowan in the “Romance of the Rail,” it was “Looping the Loop at Raurimu….” “The line is run as an ascending spiral, a complete circle (which passes over itself at a higher level), and two tunnels. The fashion in which this mountain railway ties knots in itself is rather puzzling on first experience.”

If the aeroplane had then been available to take a series of aerial photographs in the approved method of to-day for plotting contours, a complete plan of every hill, gully, plain, valley, river, and stream, with their respective heights would have shown the engineers the best location for the railway lines to avoid excessive grades, and also probable costs of construction. But such information was not available.

The Public Works engineers had for years been carrying on locational and constructional work on the line, working south from the Auckland end, and north from the Wellington end.

Owing to the wild, ravine-scarred, rugged nature of the country, the work of location between Ohakune and Taumaranui was a most difficult one, with the added disadvantage for reconnaissance work of dense bush which prevented the lay of the country being seen. Instead of air-views the intrepid engineers had to climb to the unsafe, slender topmost branches of the highest trees in the hope of obtaining an extended outlook. Finally, the line was located to Waimarino, now know as National Park Station, on the south side, and to Raurimu Station on the north side. But the former station was 714 feet above the latter one, although a straight line of only 4 1/2 miles separated them. The problem was to join up the two places with a workable grade.

From National Park a comfortable descending grade of 1 in 50 was obtained for about three miles. This left the line “in the air” 434 feet above Raurimu, and only 1 1/4 miles distant from it in a straight air line. If the railway had been constructed along that line the resultant grade would have been 1 in 15. It would then have been necessary to construct the line with a centre rail similar to the Rimutaka incline in the Wairarapa or the Rewanui incline near Greymouth.

These “centre rail” inclines are expensive in maintenance and operation. Engines of special construction are required to grip the centre rail, hence the construction of a grade negotiable by an ordinary engine was decided upon. The length required to do that was not the 1 1/4 miles existing, but one of 4 1/4 miles. Therefore, the line had to be curved, looped and spiralled downwards under itself down the hillsides until the distance along the “squirming” face, between the two points was extended to 4 1/2 miles, which gave sufficient length for a continuance of the ruling grade of 1 in 50 all the way from National Park to Raurimu. The result was a triumph of engineering skill, and a most interesting spiral line of keen interest to travellers, particularly if they take the added pleasure on the journey of seeing as they go. The skill consists— not in making a spiral, but in making it by taking captive the configuration of the surrounding hillsides and employing them so that the spiral is constructed with a minimum of embankment, cutting and tunnel, yet providing page 36 page 37 good working grades and curves of long radius easily negotiable— without retardation—by an engine and following train.

In less technical language—just make the spiral fit the country with the least expenditure of “cloth,” yet providing a good fit.

It is all very well for amateur engineers—and their name is legion—with the bush removed, and the country lying before them like an open book, to say that the spiral would never have been constructed if there had been aeroplanes in existence in those days, but I have heard engineers of standing and experience who, after looking to-day at that open book, agree that little deviation to advantage could be made in the line as located and constructed by those hardy engineers and explorers who, through almost insuperable difficulties, overcame great natural obstacles—the forest, ever seeking to conceal, the hills barring the way with their might of inertia, the deep ravines commanding “thus far shalt thou come but no farther.”

The uncanny accuracy of the work of those old-time engineers when choosing the location they did for the line, would almost lead one to think that they possessed some power equal to that of the eye of the air-camera when looking down upon the jumble of mountain and hill, and the wonderful riot of forest, tortuous ravines, and winding rivers.

The work accomplished by the surveyors and engineers before the line could be constructed was the exploration of a wide and long field of rough country to discover its configuration, the secret of which the dense bush sought jealously to hide from them. The eye of the air-camera could have swiftly revealed that secret, but the surveyors actually had to solve the problem by wearying tramps through the bush aided by the axe and the slasher until ultimately the opposition of hostile nature was overcome and the best location for the line determined.

These engineers and surveyors had blazed the trail for a line which will ever remain as a monument to their ability, sound judgment, skill, and unremitting endeavour.