In the 1940s, it became clear to the Western Australian Government Railways that the original Eastern Railway alignments were not suitable for future traffic and the loadings that were to be carried between the coast and the areas east of the Darling Scarp.

As part of the Federal Government's program to build a standard gauge line across Australia, and the passing of the Railways (Standard Gauge) Construction Act 1961 work commenced on gauge converting the line to dual gauge with a new alignment further north through the Avon Valley built with easier grades.

The Third Route was originally built with timber sleepers and 94 lb (42.6 kg) rail.
It was later upgraded using heavy (60 kg or 132.3 lb) continuously welded rail laid on new concrete sleepers during the late 1970s through into the early 1980s.

It is a dual gauge (1,435 mm or 4 ft 8 ¹⁄₂ in standard gauge and 1,067 mm or 3 ft 6 in narrow gauge) double line throughout its entirety, also including a few crossing loop sections, where there are three tracks.
These loops are located at Jumperkine, Moondyne and Toodyay West.
Numerous cuttings were also constructed, including the deep Windmill Hill Cutting east of Toodyay.
A major marshalling yard is located five kilometres west of Northam.

This new route opened on 14 February 1966 and coincided with the closure of the earlier two routes of the Eastern Railway.
In the 1940s, it became clear to the Western Australian Government Railways that the original Eastern Railway alignments were not suitable for future traffic and the loadings that were to be carried between the coast and the areas east of the Darling Scarp.
As part of the Federal Government's program to build a standard gauge line across Australia, and the passing of the Railways (Standard Gauge) Construction Act 1961 work commenced on gauge converting the line to dual gauge with a new alignment further north through the Avon Valley built with easier grades.
The Third Route was originally built with timber sleepers and 94 lb (42.6 kg) rail. It was later upgraded using heavy (60 kg or 132.3 lb) continuously welded rail laid on new concrete sleepers during the late 1970s through into the early 1980s.
It is a dual gauge (1,435 mm or 4 ft 8 ¹⁄₂ in standard gauge and 1,067 mm or 3 ft 6 in narrow gauge) double line throughout its entirety, also including a few crossing loop sections, where there are three tracks. These loops are located at Jumperkine, Moondyne and Toodyay West. Numerous cuttings were also constructed, including the deep Windmill Hill Cutting east of Toodyay. A major marshalling yard is located five kilometres west of Northam.
This new route opened on 14 February 1966 and coincided with the closure of the earlier two routes of the Eastern Railway.
Eastern Railway (Western Australia)