Knapsack Viaduct was completed in 1867 as part of the Lapstone Zig Zag Railway. At the time, it was the tallest structure in the country and visitors would flock to the site to marvel at its construction. The bridge was abandoned in 1913 when the rail line was diverted around the zig zag. In 1926, the bridge became a key part of the Great Western Highway, and was widened in 1939 to cope with extra traffic. To help maintain the original design and style, stone columns and arches were added to each side of the viaduct to support a wide concrete slab laid on top of the original carriageway.