The Knapsack Viaduct was designed by and built by John Whitton. Its purpose was to carry the Railway Line across Knapsack Gully. It formed part of the Zig Zag, which climbed the eastern escarpment from Emu Plains to today's Glenbrook.
Construction of the Knapsack Gully Viaduct started in March 1863 by contractor W. Watkins, who also completed the stone piers of the Victoria Bridge at Penrith. Work was completed in 1865, with the bridge being constructed from local sandstone quarried in the neighbourhood around Lapstone, and carried a single rail line.
The viaduct fell into disuse in 1913 after the completion of the Glenbrook Gorge Deviation. In 1926, after over a decade of disuse, the Knapsack Viaduct was taken over by the Main Roads Board. The board sort to improve the route of the Great Western Highway between Emu Plains and Blaxland, that at the time zig zaged up what is today Old Bathurst Road. The viaduct's carriageway was widened to allow for two car lanes, by trimming back the inside face of the stone parapets. The new road was opened by Governor Sir Dudley DeChair on 23 October 1926. The viaduct was again widened in 1939, with the construction of a reinforced concrete cantilevered deck, because of increased traffic use.
The final deviation of the Great Western Highway, with the opening of the M4 finally ended traffic flow across the Knapsack Viaduct in 1993. In 1995 the Viaduct was reopened to the public as part of the historic Lapstone Zig Zag walk.. From Wikipedia