Where road trains are legal there are geographical limits on where they're allowed. You'll find that outside most cities, just at the point where you start to see buildings, is a truck yard where they swap tractors or just take a trailer off and put it onto another truck. Where the roads are long and straight road trains are fine, but they don't corner very well.

The economics are a bit dodgy - they rely heavily on subsidised roads and cheap fuel, but for many remote communities it comes on a truck or it doesn't come at all. Mind you, the cost for the Alice to Darwin railway line makes the road cost seem like pocket change. And no, actually, road trains are still the dominant way to get stuff from Darwin to Alice.

One odd thing in Oz is that there are private railway lines - some of the iron and coal mines have hundreds of kilometres of railway line between the mine and the sea, all on private land. Their roads can also be exciting - they'll sometimes drive the smaller mining vehicles on them (and yes, a 100 tonne mining truck *does* have the right of way).