>>1427293>loading gauge and structure gaugeCrap, I hadn't thought of that. Obviously it would be easy enough to convert to standard gauge in Victoria and SA with a loading gauge that can accommodate single deck trains across all of NSW/Vic/SA. The restriction to single deck isn't a huge deal. But trying to convert the underground bits of rail in Brisbane and Perth might very well be nigh impossible unless they were overbuilt to begin with. Still, I think this can be worked with.
Western Australia is already quite isolated from the rest of Australia due to the desert areas and that isn't likely to change unless we figure out terraforming, so keeping it narrow gauge should be fine. We're also kind of lucky in that Brisbane, an ideal place for an unobtrusive break of gauge, is so close to the Queensland border. There aren't even many stations on the north coast NSW line between Coffs Harbour and Brisbane itself, since the rail corridor is too far inland compared to the towns in the area. Pic related. And also conveniently, there's a bigger difference between 3'6" narrow gauge and standard gauge than there is between standard gauge and 5'3" broad gauge which makes dual gauge rail easier.
So we should convert all broad gauge to standard gauge, build an eastern seaboard maglev line at least as far north as Brisbane using the route mentioned previously, then start terminating low/medium speed passenger services at Coffs Harbour. That leaves only NSW<->Qld freight to deal with. Some of the proposed future expansions of the Inland Rail project show that's still going to be a bit of an issue with a fair bit of dual gauge around, but I'm not really sure what else could be done there.
