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The Hawkesbury River railway bridge carries the Main Northern railway line across the Hawkesbury River, connecting just north of the town of Brooklyn, Hornsby Shire on the northern outskirts of Sydney with Cogra Bay, Central Coast Council, New South Wales. The railway bridge was to be the last link in a railway network that linked Adelaide, Melbourne, Sydney and Brisbane and was a major engineering feat at the time.
The contract for the original bridge was awrded to Union Bridge Company of New York in January 1886. The bridge officially opened on 1 May 1889.
Numerous problems arose, and in 1938 a severe crack in one of the piers was discovered and it became necessary to replace the entire structure. The amount of traffic being carried during World War II made the replacement extremely urgent.
Design and construction of a replacement bridge commenced in 1939. The design, foundation work and fabrication of the new bridge were undertaken by the New South Wales Government Railways.
The above information is condensed from the Wikipedia entry.
Regarding the severe crack in No.4 Pier in 1938, Major-Gen Albert Cecil Fewtrell of the NSW Railways found in an American tex book that the interior of the piers of the bridge comprised rubble, and noted this was contrary to the specification, which required concrete to ensure stability when the iron casings rusted. Fewtrell maintained that he was responsible for identifying this defect and avoiding a major rail catastrophe.