RAILWAY CONSTRUCTION. 103 



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 ornamenta- 

 tion 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. 



