To carry out the undertaking, the engineer has to prepare working plans and sections to a somewhat larger scale than that adopted for the Government or Parliamentary plans, and on which must be marked the exact positions of the commencement of the curves, straight lines, and gradients. The sites of all the over and under bridges must be shown, and their angles of crossing noted. All road, river, or stream diversions must be indicated, so that the work in connection with them may be laid out on the ground. All culverts and drains must be marked, and their size, depth, and direction described. Public road level-crossings, and farm or occupation-road crossings, must be shown in their proper positions.
The face-lines of the ends of all tunnels should be marked on the working plan and section, and the position of any shafts, which may be intended either for use in carrying on the work or for future ventilation.
A considerable amount of investigation and negotiation will have to be entered into before the locating of the above works can be finally decided. The desire to meet the wishes and convenience of all parties interested must of necessity be controlled by the physical circumstances of each case; very little alteration can be made in the level of the rails, although some variation may be made in their position.
When fixing the depths of culverts and drains, attention must be paid to any probable improvement in the drainage of the district, which might at some future time necessitate the deepening of such of the main culverts where the inverts had been laid too high.
Unless all these details are determined, and shown on the working-plans before the works are commenced, there is the risk that embankments may have to be opened out to admit of bridges and culverts, and cuttings changed to permit of road diversions.
The entire centre-line of railway must be carefully staked
out by driving strong wooden pegs into the ground at the end of every chain length, and along the course of these pegs the longitudinal section must be taken. Three pegs, one on each side of the centre peg, are generally placed at the commencement and termination of the curves. When the longitudinal section has been plotted to scale, and the course of the gradients and level portions worked out and drawn on, then the heights of the ground level and formation level can be marked at each chain, and from them the depths of the cutting and the heights of the embankments can be ascertained and marked at each chain. In addition to the longitudinal section, it will be necessary to take a large number of transverse or cross sections at those pegs, or intermediate points, where the ground is at all side-lying or irregular. These cross-sections are necessary to determine the side-widths, or distances to outer edge of slopes in cuttings or embankments, and also to calculate the actual quantity of earthwork to be executed. For convenience in taking out the quantities, these cross-sections are generally plotted to a natural scale, that is to say, to the same scale horizontal as vertical, as shown in the example of cross-sections, [Figs. 15 to 24]. It is also necessary to obtain information, by boring or otherwise, as to the material of which the cuttings are composed, whether clay, gravel, or rock.
In laying out lines through fairly level plains and populous districts, the absence of great natural obstacles will allow the engineer to carefully consider how far it may be prudent to diverge to the right or to the left, to accommodate towns and places which would be excluded by a more direct through route. There will be ample range for selection, and it will be rather the question of policy than compulsion which will guide him in the route to be taken.
