The New Zealand Railways Magazine, Volume 6, Issue 1 (May 1, 1931)
Railway Location Methods
Railway Location Methods.
Surveying in Relation to Railway Engineering.
(See accompanying letterpress for explanatory particulars.)
The modern practice is to compensate railway grades for curvature, i.e. to provide a slightly easier gradient on sharp curves to balance the resistance of wheel flanges on the rails on such curves.
Railway location in rough country calls for considerable investigation and trial lines. To reach a given elevation by a direct route is often impracticable as the gradient would be prohibitive. The only alternative to get a working gradient is to cover distance, called development. The Raurimu Spiral is an example of this. It is frequently remarked by railway passengers, when proceeding up a valley and they see the river well below them, that they cannot understand the mentality of the engineers in leaving the river flats where the gradient is so easy. They overlook the fact that in the lower part of the valley the river has an easy gradient, but higher up the valley it begins to rise until finally it may be as steep as 1 in 10. The railway leaves the level of the river at the lower entrance of the valley and following a workable grade throughout eventually again joins the river level at its source and passes over the summit to the adjoining watershed. This is illustrated in figure No. 18.
In past years it was the practice to go in for cheap construction, the alignment and grades following the contour of the country. This gave heavy gradients and sharp curves, but light banks and cuttings. The Lawrence Branch is a typical example. This practice is economically unsound. Though the initial cost is light, the limitation of loads, speeds and the heavy wear to track and rolling stock is perpetuated and this expense would more than pay for a better and more costly location in the first instance. When traffic demands make it imperative to improve the old location by grade and curve easements, the money expended on the original location may for the most part be written off as a dead loss. In locating and grading a line, the likely trend of heavy loading should be investigated. To have an average falling gradient over many miles in the direction of heavy loading, with a steep opposing gradient for a short distance at some point in this area is bad practice. The opposing grade limits all loads on the down gradients to that of the up gradients.