RAILWAY CONSTRUCTION. 133 



approach, the greater portion of the wing walls will have to be 

 built up from the solid or original ground, and there will be 

 a large amount of masonry below the finished ground line, as 

 indicated in Fig. 128. 



In some cases of over-line bridges it is necessary to curve the 

 wing walls to correspond to the road which turns off to the 

 right or left after crossing the railway, as shown in Fig. 129 ; 

 or the wing walls may have to form two separate curves where 

 the road branches off in two directions after leaving the bridge, 

 as shown in Fig. 130. 



Fig. 131 shows plan, elevation, and cross-section of an under- 

 line arch bridge, considerably on the skew, carrying a railway 

 over a river. The wing walls are curved, and very similar in 

 type to some of those in preceding examples. The river bed 

 and ground alongside being of solid rock, good foundations were 

 obtained at a very moderate cost. 



On many railways constructed in the beginning as single 

 lines only, the over-line bridges have been built for double line. 

 The additional cost in the outset has been small, compared with 

 the great expenditure which would be incurred afterwards in 

 reconstructing the bridges to suit a double line. 



The general arrangement of abutments and wing walls shown 

 in the foregoing examples will apply to similar classes of bridges 

 where girder- work is adopted instead of arching. 



There are many ways of forming the floor or deck of a girder 

 bridge intended to carry a railway over a road or stream. In 

 some cases it will be imperative to have a thoroughly water-tight 

 floor to prevent rain-water percolating through to the roadway 

 below; while in others, such as bridges over streams, and 

 secondary roads, this special provision will not be necessary, and 

 a lighter and more economical floorway can be adopted. A 

 strong wrought-iron or steel-plate flooring, with its correspond- 

 ing filling and ballasting, means not only so much additional 

 cost in the flooring proper, but also so much additional dead 

 weight to be carried by the main girders. 



Fig. 132 is a sketch of rolled joist-iron I-girders and timber 

 floor frequently adopted for small farm roads and cattle creeps 

 of 10 or 12 feet span. A beam of timber is fitted in between 

 the two rolled joist-irons, and the three pieces securely fastened 

 together with strong iron bolts placed about 3 feet apart. These 

 small compound girders rest on bearing- plates of wrought or cast 



