The dual gauge in Western Australia runs from Avon Yard (West Northam) through to Kwinana in the south-west of Perth - approximately 160km (100 miles) of dual gauge, with around 90% of it dual tracked. The images below are between Toodyay and Northam showing both gauges being used, and this line sees around 30 trains per day (1/3 narrow gauge roughly).
This shows the Mineral Resources iron ore train from Koolyanobbing to Kwinana on the standard gauge
This image shows a loaded narrow gauge ARG (then QR National, now Aurizon) grain train running to Kwinana for export from the CBH facility.
This image shows the difference well, with the loaded ore train (left) on standard gauge heading to port through Forrestfield, and the empty grain train (right) heading back to Midland and then Moora on the Geraldton line (narrow gauge only to Geraldton).
Cheers,
Matt