Of _bridges_, over 200, with spans ranging from nine feet to 130 feet, were destroyed wholly or in part during the progress of the war; but even here the speedy restoration of traffic did not, as a rule, present any very grave difficulty. The course generally adopted, as one suited to South African conditions, was, not to start at once on the repair of the damaged bridge, but, in order to meet the exigencies of the moment, to construct a diversion or deviation line alongside, with small low-level bridges on piers, built of sleepers and rails.[41] These deviation lines offered great disadvantages by reason of their sharp curves, their steep approaches and their liability to be washed away in wet weather. The building even of temporary bridges across deep rivers having a considerable volume of water also caused inevitable delays. But the lines in question served their purpose until the reconstruction of the damaged bridges--taken in hand as speedily as possible--could be effected. Anticipating the needs for this more permanent work, the Director of Railways had arranged before leaving England for a supply of girders, similar to those in use in South Africa, to be sent out, together with sufficient timber, of useful dimensions, to rebuild the whole of the railway bridges in the Orange Free State, should it become necessary so to do--as, in point of fact, it did. Of new rails he had available, at one time, a total length of 300 miles. By October, 1900, the makeshift repairs completed on all the lines taken from the enemy were being gradually converted into permanent or semi-permanent reconstruction by the Works Department of the Imperial Military Railways; but the continuous guerilla raids of the enemy still made it impossible to run trains by night. These conditions led to a resort to the system of _blockhouses_ which, first constructed for the defence of railway bridges in Natal during the advance for the relief of Ladysmith, and used extensively when Lord Roberts marched from Bloemfontein into the Transvaal, leaving a long track of railway lines behind him, were subsequently so far extended that the whole of the railway lines in the Transvaal and the Orange River Colony were provided with them.[42] So well did they answer the purpose that by April, 1901, the worst of the trouble involved in maintaining railway communications was over, although another year was to elapse before peace was restored. MILITARY TRAFFIC An especially remarkable achievement with which, under the various conditions here narrated, the Department of Military Railways is to be credited was in connection with the concentration of the force with which Lord Roberts marched from the Modder River to Bloemfontein. The movement began on January 21, 1900, by which time the repairs of the lines had been completed, and within three weeks no fewer than 20,000 men, 13,590 horses and over 24,000 tons of stores had been conveyed over a single line of railway.