RAILWAY CONSTRUCTION. 141 



placed the ballast and permanent way. Brickwork might have 

 been used for the jack-arching, but concrete was considered more 

 convenient. 



Fig. 144 shows the cross-section of a truss-girder bridge of 

 123 feet span, carrying a double line of railway over a wide 

 thoroughfare, the load being placed on the lower flange. There 

 are two main girders, each 12 feet 6 inches deep in the centre, 

 and 8 feet deep at the ends. Plate cross-girders are placed at 

 4 feet 6 inch centres, on which is riveted longitudinal plate- 

 iron troughing, extending across the bridge and terminating at 

 the sides with wing-plates, as shown. The entire floor is 

 covered with a thick layer of asphalte previous to filling in 

 with ballast to receive the permanent way. Plate stifFeners are 

 adopted in this bridge very similar to those in Fig. 139. 



Fig. 145 gives plan, elevation, and cross-section of a plate- 

 girder bridge of 95 feet span, carrying a double line of railway 

 over a very busy street. There are two curved-top main girders, 

 each 10 feet 9 inches deep in the centre, and 6 feet 7^ inches 

 deep at the ends. The arrangement of cross-girders, longitudinal 

 plate-iron troughing, and permanent way, is very similar to that 

 in the preceding example, but the side wing-plates are carried 

 up higher, and are riveted up to the web-plate of main girder, 

 forming continuous stiffen ers from end to end of the main 

 girders. A light, ornamental, close cast-iron parapet is bolted 

 on to the top of the curved, or upper, boom of the main girder, 

 the top line of the parapet being carried out parallel to the 

 bottom boom of girder. This bridge crosses the street very 

 obliquely, and, although cast-iron columns were allowed at the 

 edge of the footpaths, the main spans are unavoidably large. 

 When designing the above bridge, the writer had to adopt a 

 girder that would form a screen, to provide a deck, or floor-way, 

 which would be not only water-tight, but also deaden as much 

 as possible the sound or vibration of passing trains, and at the 

 same time give some ornamental appearance to the girders and 

 parapets. This bridge carries a constant service of heavy trains ; 

 it is perfectly dry underneath, and is remarkably free from 

 noise or vibration. 



Fig. 146 shows cross-section of a plate-girder bridge of 

 40 feet span, carrying a double-line railway over a street, in a 

 situation where the depth from top of rails to under side of 

 girders had to be made as small as possible. Three main girders 



