The sandstone viaduct was designed by John Whitton and built over the period 1863-1864 to bridge the Knapsack Gully on the first railway route over the Blue Mountains. It was the largest viaduct in Australia being 388 feet long, 120 feet high with five spans of 70 feet and two of 20 feet. It was hailed and a engineering feat by our colonial forebears. It was a landmark for nineteenth century travellers to the Blue Mountains and beyond.
It is a massively impressive structure especially from the gully below. Taking the path from the northern side of the viaduct, we descended below to the creek bed where we
could stand next to the sandstone arches. Then taking the stairs up on the southern flank gives further interesting angular views of the viaduct. Further along is the Knapsack Quarry from where the sandstone was obtained for the construction of the viaduct. From here you can ascend to the Elizabeth Lookout and Zig-Zag Railway.
When the railway was re-routed through the Glenbrook Gorge in 1912, the lower section of the older track including
the Knapsack Viaduct was converted into a road, the Great Western Highway. This was the main road up the Lapstone until the M4 Motorway replaced it in 1993. The viaduct was widened in 1938 by moving the stone parapets outwards and placing them on concrete cantilevered slabs.
History Services NSW in its Government Contracts and Contractors database has many entries for railway contracts awarded by the NSW Government for the period 1832-1900.
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