the mine is 25kms from the alice to darwin rail line. the mine is expected to be the longest running salt mine in australia so why not build the 25kms branch to the mine site and be done with it. no double handling?
The cost of building the main line is shared amongst all its users and the governments. The cost of building a spur is shared by no-one but the user.
The volumes handled along that spur are not large. You could buy a lot of double handling (and more expensive road transport) for the cost of a 25 km spur.
Those two factors alone probably justify the proposed setup alone. But further - if the cost of double handling and the higher cost of road transport made it worthwhile, you can always decide to build the spur later. Once you've built the spur, if things don't work out then you cannot generally turn around and ask for your money back. So there's no need to rush.