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From the Oxford English Dictionary: "An elevated structure, consisting of a series of arches or spans, by means of which a railway or road is carried over a valley, road, river, or marshy low-lying ground."

There are a number of sandstone viaducts of masonry arch construction on either side of the Blue Mountains and on the western side.

When the railway was built there were only three materials for bridges:

Timber - quick to build but short lived because it was vulnerable to termites and rotting.
Iron - long lasting but expensive as it had to be imported from UK, and took months to arrive.
Sandstone, long lasting, but expensive unless locally available which it is in the Blue Mountains.

Lithgow Zig Zag has three viaducts.

Others are:

Knapsack Gully Viaduct, 1863-1867. On the lower side of the Lapstone Zig Zag there is a fine sandstone viaduct over Knapsack Gully, used as a road bridge on the Great Western Highway after the line of the railway was changed in 1892. (388' long, 120' high, 5 spans of 55' and 2 of 20'.)

Lennox Bridge 1833. A fine sandstone road bridge named after its builder David Lennox, not far from the Knapsack Gully Viaduct.

Lithgow, 1869. Underbridge for James Street. (3 x 20' arches). Oldest viaduct in use on the Western Line.

West of Lithgow:

Farmer's Creek Viaduct,1870,(7 x 30' arches). Farmers Creek viaduct needed strengthening soon after construction, when wrought iron reinforcing was added. It was doubled by the addition of a brick viaduct (6 x 48') in 1921.

Middle River Viaduct, Marangaroo, 1870, (8 x 30' arches), replaced by a brick viaduct (4 x 48' arches) in 1921. Pumping station chimney built into viaduct 1920-21 for porvision of locomotive water supply at Wallerawang. A trout farm is situated at its base.

Cox's River Viaduct, Wallerawang, 1870, (7+1+10 arches, middle arch over the river 54' wide, the others were 25', c. 500' long). The only large span railway arch bridge, the structure is unique in Australia. Replaced 1923 by brick and steel parallel viaduct.

Further west along Solitary Creek there are 6 more stone arch viaducts.

From Robert Lee's book, there were 7 viaducts in the contract between Clarence and Wallerawang. 3 at Zig Zag, plus 4 more.

Updated 16 November 2008