'58 



RAILWAY CONSTRUCTION. 



entire site of the work is at the disposal of the erectors, and the 

 building of a bridge or viaduct can be carried on with a freedom 

 which cannot be obtained on an open line. On a working 

 railway, the train service must be kept going, irrespective of 

 renewals, and very often the best that can be done is to reduce 

 the double line to single line working at the site of the operations. 

 It is not always expedient or possible to make a temporary 

 bridge and diverted line for traffic purposes, as the expenditure 

 to be incurred might be too great to warrant the outlay, or there 

 may be local difficulties to effectually prevent the introduction of 

 a provisional structure. The taking down of one half of the old 

 structure may necessitate the removal of stays and bracing 

 affecting the stability of the half remaining to carry the traffic, 

 and thus render temporary shoring and bracing necessary. The 

 erection of the new work in such a limited space has to be 

 watched with great care; all cranes, lifting appliances, and 

 scaffolding must be kept clear of vehicles moving over the 

 running-line, and very frequently it is found prudent to cease 

 erecting operations during the passage of a train. 



In very many cases of renewals, the description and arrange- 

 ment of the old structure will materially influence or control the 

 design for the new one, and the details of the latter must be 

 schemed out so as to disturb as little as possible the stability of 

 the old work remaining as the working road. 



The following list gives the lengths of the main spans of some 

 railway bridges, and may be found useful for reference : 



LENGTHS OF MAIN SPANS OF SOME LARGE BAIL-WAY BRIDGES. 



