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The New Zealand Railways Magazine, Volume 6, Issue 1 (May 1, 1931)

Railway Location Methods

Railway Location Methods.

When locating a railway or road, short lines of levels are taken at each chain at right angles to the line of proposed construction. These lines are called cross sections and are used to determine the best exact location of the centre line of railway or road for economy in construction and at the same time to give data from which the filling or cutting at each chain can be computed. In figure No. 17 A are shown typical cross sections with the filling or cutting shown thereon in dotted lines, the heights of such filling or cutting being obtained from the plan of the levels and grades, known as the longitudinal section, as shown in figure No. 17. This figure shows a small portion of the finished plan for the construction of a railway. Above is the alignment or location of the centre line, with curve. Below is a graphic representation of the mileage at every chain, the heights of the surface at every chain, the heights of the formation decided upon, with a grade of 1 foot per chain, i.e., in this case a fall of 1 foot in each chain. The difference between the surface heights and the formation height give the heights of the filling or cutting at each page break
Surveying in Relation to Railway Engineering. (See accompanying letterpress for explanatory particulars.)

Surveying in Relation to Railway Engineering.
(See accompanying letterpress for explanatory particulars.)

page 44 chain. The surface and grade heights are plotted, vertically to an exaggerated scale, to bring the variations more prominently under the eye. Three cross sections are shown at mileages 101 M. 74 Chs., 191 M. 79 Chs. and at 102 M. 2 Chs. The slopes of earthwork shown are the usual practice. The heights of the filling or cutting shown on the sections are taken from the longitudinal plan, as will be noted. Before the final location and plan is made trial computations are made so that the cutting will approximately equal the filling in the immediate locality, as it is not economical to have cutting to spare or to have to borrow filling. To bring about an approximate balance the final location is thrown a little further into a hillside if more filling is wanted, or a little further from the hillside if there is a surplus of cutting, or as an alternative the grade is altered, if practicable.

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.