Some time soon, exactly when I'm not sure, the Alice Springs to Darwin rail will open and it will finally be possible to visit all the mainland capitals by rail.
The operators of the Indian Pacific, Sydney to Perth, who also do the Overland, Melbourne to Adelaide, will be operating the Darwin train.
I'm not sure if it has been named yet, or whether it is just an extension of the Ghan, which runs from Adelaide to Alice Springs.
In any case with a truck load of money and some study of the timetables you can do Brisbane-Sydney-Canberra(I'm not sure why but some people do go there!!)-Melbourne-Adelaide-Darwin-fly back to Adelaide then rail to Perth.
It's a mongrel of a life but someone has to do it!!