It's not the fault of the road or rail traffic. It's the clay soil that is the problem. Swells and contracts with the moisture (or lack of) and causes all the undulations in roads and the track. 80km/h is the rail max, often 40 -60 in places. Definitely no difference being standard gauge - but it would be advantageous.
If the line went from Mildura northwards to the east-west line, quicker access to markets for Mildura produce. And wouldn't be locked out for a couple of weeks due to derailments!