Don't get too hopeful. Today's commitment is only $95m, which is around 1/6th of the project cost. It's basically what you would call "seed capital".
That prospective light rail network I posted elsewhere with 3 limbs at either end I would estimate is around $4-5bn in today's dollars, excluding the extension of the heavy rail to the airport.
The major advantage of GCLR over any other project floating around is that it is the easiest thing to just tack modest extensions onto over time. CRR, NCL, MM etc are megaprojects costing multiple billions, and metropolitan rail projects elsewhere have wide-ranging consequences for the existing network that need to be sorted out. Probably the only other one which is easy enough to commit to without doling out a huge chunk of money would be finishing the Gawler line electrification.
This is a good step but it isn't evidence that they are going to be very free with the purse-strings - after all, proportionately this is only around half of the federal contribution for Stage 1.