The steel lattice-girder Koilwar Bridge (known as Sone Bridge when it was built) was the longest bridge in the subcontinent when built: construction was started in 1856, disrupted by uprisings in 1857, and completed in 1862. A two-lane road (NH 30) runs under the twin rail tracks.
George Turnbull's 1851 diary of four-night-and-day journey to Sone River and survey there.
A page from George Turnbull's 1851 notebook detailing his determining the approximate width of the mile-wide Sone River at the point where he decided that the bridge should be built.
An initial survey of the bridge site was made on 17 February 1851 by George Turnbull, Chief Engineer of the East Indian Railway Company: he determined that the river then was 5,350 feet (1,630 m) feet across — the completed bridge was 5,280 feet (1,610 m) feet across. He settled on the site near Pures "where the banks are well defined, and the channel had evidently for ages been confined within certain limits, proved by the existence of old Hindoo temples, far before the Mohammaden works at Muneer, built about 200 years [before 1851]."
By November 1859, both abutments and 16 of the 26 piers were being built and the well-sinking for the remaining piers progressing. By 21 December 1860, three of the iron spans were in place; 4572 tons of the estimated 5683 final tons of iron-work for the bridge had arrived from England.
George Turnbull inspected the bridge and judged it complete on 4 November 1862. On 11, 12 and 13 December 1862, "a set of experiments with couple engines, testing the Keeul, Hullohur and Soane bridges, with an assembly of Government engineers, and our railing engineers; all very satisfactory." On 5 February 1863, a special train from Howrah took Turnbull, the ViceroyLord Elgin, Lt Governor Sir Cecil Beadon and others over two days to Benares: they alighted at the bridge and inspected it. In Benares there was a durbar on 7 February to celebrate the building of the railway and particularly the bridging of the Sone, the largest tributary of the Ganges.
Sand erosion near the pillars of this old bridge has created structural problems recently.