In Fig. 102, the cutting being deep, almost up to the level of the public road, the foundations of the wing walls are built in steps, resulting in a minimum of masonry below the finished ground line. Where the cutting is shallow, and the public road has to be brought up to the bridge on an embanked approach, the greater portion of the wing 133 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 corresponding 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. [Illustration: Fig. 132, 133, 134, 135, 136, 141] 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 iron, and are held 135 together and to gauge by tie-rods, as shown. The rails are spiked or bolted down on to the timber beams, and the flooring formed of strong planking.