Absolutely, but all the track was laid when each state ran things their own way. Victoria chose mostly broad gauge, NSWGR went standard, QLD went narrow and SA couldn't make up their minds and chose all three.
All the interstate main line is standard and only a few remaining branch lines in SA and VIC are broad gauge and there's no money in converting them. I think QLD still run a fairly big narrow gauge network but I dont know much about that area.
It's been a great way to see the country and get paid for it. The railways arent what they used to be here and as I said it's amazing to see some of the old infrastructure, see big old abandoned yards which thirty years ago were bustling metropolises of activity and action.
Here's Delec (a very big and important Sydney depot in its day) as it looked 30 years ago:
And here it is now:
(not my photos)
So I dunno, this is just one example and it can be quite chilling wandering around an abandoned yard such as in Cowra.