Railway bridges over or under public roads of primary or secondary importance must be constructed to the widths and heights prescribed for such works in the fixed regulations of the
country in which they have to be built. As a rule, these road-bridges are simple and inexpensive in character, except in towns, or in cases where the line crosses the roads very obliquely, or where the road is situated at the top of a deep cutting, or bottom of a high embankment. Away from towns and out in the open country, permission is generally obtained to divert the roads to a moderate extent, so as to obtain a more favourable angle and height for the bridge; but in towns, where the roads become streets, sometimes of great width, with houses and shops on each side, little or no diversion can be allowed.
A railway passing through a portion of a densely populated town must deal with the streets as they exist, as any great alteration in their course or continuity would involve a large destruction of property. With careful laying out it is possible to obtain favourable crossings for many of the streets, but a number of others must be crossed obliquely, and these oblique crossings very frequently result in a span twice the width, or even more, of what would be necessary to cross the street on the square. Bridge-work in towns is more costly than in the country, as a higher class of work is demanded, more finish or dressed work in the masonry or brickwork, and more ornamentation in the screens and parapets in connection with the iron girder-work. The work itself has to be carried on in a confined locality, with limited space for materials and appliances, and where the thoroughfare must be kept open.
Where the height is sufficient, and suitable materials readily obtained, it is preferable to adopt an arch bridge, as being of a much more permanent character than girders.
[Fig. 102] is an example of an ordinary over-line arch bridge to carry a public road over a double line of railway in a cutting of moderate depth.
[Fig. 103] shows a somewhat similar over-line arch bridge, but its height from rail to road-level being greater, side arches are introduced in preference to long heavy wing walls.
[Fig. 104] shows an over-line arch bridge in a rock cutting. In this case, by increasing the span and forming the springing bed in the solid rock, the masonry of abutments and wing walls may be reduced to a minimum.
[Fig. 105] is a sketch of an ordinary under-line arch bridge to carry a railway over a public road in an embankment of moderate height.
