Interesting tussle between the port owner wanting to access the Darwin to Adelaide traffic and the rail owner wanting more trains on their line.
So that will be negotiated. Normally the line owner would say to the source generator (like a mine) to pay for any line extensions, but they wouldn't want that traffic going to road. However as it is mooted as a government port, they will probably pick up the tab.
Long way to go on this. The only reason that this has any legs is the strategic value, certainly not due to capacity constraints.