Mick, the rail lines in those areas are single track too, and I assume they have passing loops although I've never been in that part of Australia. I imagine the trains are long so as much iron ore as possible is moved as quickly as possible with as little man power as possible.
My daughter and son-in-law both work in the coal industry in the central highlands of Queensland, she is an O.H. & S. advisor, and he is a mine engineer. We visit regularly and see the big coal trains operating in that area. Each train has 2 locos, 50 wagons, another 2 locos and another 49 or 50 wagons, each wagon holds about 80 tons of coal so there is a fair load all up. The trains operate a merry-go-round system between the mines and the port loading facilities on the coast, or a big power station and, at the peak there seems to be a full train every half hour or so, with the empties running in the opposite direction.
In the Hunter Valley of NSW, however, there is an even bigger operation hauling coal from the Valley mines to the biggest coal loading facility in the world, at Newcastle. Multiple lines run into the loaders, and it's amazing to watch the huge bulk carriers being loaded, then making their way through the river mouth to the open sea assisted by 3 or 4 tugs.
Peter.