Perhaps one of the most anxious tasks which falls to the lot of an engineer is the renewal of under-line bridges and viaducts on a working line. On a new line in course of construction the entire site 158 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 arrangement 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 RAILWAY BRIDGES.