Chalker Parade Railway Overbridge, Hill Top

An "Overbridge", in NSW railway parlance, carries some other service over the railway route. An "Underbridge" is a bridge which carries the railway track.

This bridge is significant in that it is the first reinforced concrete bridge to carry road or rail traffic in New South Wales and one of the earliest in Australia, if not the first. As a reinforced concrete structure using the Monier system, it is predated by a year by the sewerage aqueducts built over Whites Creek and Johnstons Creek in Sydney, but the more onerous loading condition at Hill Top, of a heavy rolling load such as a traction engine or wagon was an extra challenge for the designers – the engineers of the NSW Railways.

The arch has a span of 40 feet (12.2m) and a rise of only 4 feet (1.2m). With such a small rise there is a risk that under some loads unacceptable tension could occur in the concrete of the arch ring leading to damage or collapse. This is countered by casting into the concrete, near both faces of the curved slab, small (6 and 10 mm) steel or iron rods, much as would be done in modern concrete work. The slab of the arch is very thin at 8 inches (200mm) and is only slightly thicker at the abutments.

The Monier technique was pioneered in the 1860s by Joseph Monier, an illiterate French gardener, to make flower pots. There were other techniques for reinforcing concrete being tried at the time of this bridge’s construction but the Monier method of using a mesh of relatively small bars became the dominant method in the twentieth century.

The bridge was designed to span two tracks as at the time it was planned that the railway line below would remain as the Main South Line, but once the interstate route was diverted through Bargo in 1919 the cutting below the bridge was never widened.