[Fig. 73] gives an example of the syphon arrangement as constructed with two cast-iron pipes placed side by side.
Railway works carried out in cities and large towns, whether they take the form of cuttings, embankments, arching, or tunnels, are certain to cause a very considerable disturbance of existing drains, corporation sewers, gas-pipes, water-mains and underground telegraph wires. Some of these underground works may be so peculiar and complicated as to necessitate a slight deviation from the course originally intended for the line. Suitable provision will have to be made for each of the items interfered with by the railway, and the substituted work must be carried out to the satisfaction of the constituted authorities within the municipal boundaries.
Bridges.—Amongst the many bridges and viaducts which have to be built during the making of a railway those constructed over rivers and waterways are generally the most important
The bridging across any navigable river or tidal water can only be effected in compliance with conditions imposed by the authorities controlling the navigation rights. These conditions will place restrictions as to the number and distance apart of the piers, as well as the height from high water level to the under side of the arches or girders. For rivers having a constant traffic of sea-going vessels of large tonnage and lofty masts the authorities will demand great height or headway as well as large spans; and if to this be added a deep water-way and bad foundations, the work to be constructed becomes one of considerable magnitude. The banks of the river must be carefully studied to find the most favourable point for crossing, and in some cases it may be prudent to make a detour of two or three miles. The crossing at a great height involves the construction of the approach lines at a great height also. If the river is in a deep valley with high sloping sides the natural contour of the ground facilitates the formation of the approach lines; but with a river on a low, wide, open plain, inclined approach lines add enormously to the cost of construction, as well as to the cost of permanent working.
If the number of sailing craft passing up and down the river be moderate, and, perhaps, only passing at high water, the authorities may permit a low-level viaduct with an opening bridge.
There are thus the two systems: the high-level viaduct, which allows trains to pass over and vessels to pass under at any and all times, and the low-level viaduct with opening bridge, which, if open for vessels, is closed for trains, or vice versâ.
Every crossing of a navigable river will have to be considered and dealt with according to its own individual requirements. An arrangement suitable for the one may not be admissible or prudent for the other. A frequent and important train service might be much interfered with by an opening bridge, and, in a similar manner, an opening bridge might cause much interruption and detention to the navigation of the vessels on the river.