Your post was all going fine until you mentioned Adelaide to Victor via Mt Barker. Who would use this service? I'm not sure if you know, the quickest way to Victor is through Seaford. The quickest way to Barker is down the SE Freeway. The train takes neither of these routes, instead going via Blackwood, Heathfield, Balhannah and Littlehampton, and then slowly making its way to Victor via bloody everywhere. Steamranger currently runs Mt Barker-Victor services, which are tourist trains only.
I think that a small regional network could maybe run. Adelaide-Whyalla via Pt Pirrie, Adelaide-Broken Hill and Adelaide-Mt Gambier ""might"" be sustainable if they only ran 3 times a week in each direction at most.
Other states may have regional rail networks, but just look at the population distribution in SA. Out of 1.5million, 1.2 live in Adelaide. That leaves 300,000 spread out in a very large area. If Mt Barker keeps growing the way it is, it will be SA's second City in no time.
What really needs to happen in SA is create a new rail corridor to Mt Barker, following the freeway alignment as much as possible. Electrify it, make it express, stopping only Mt Barker South-East, Mt Barker Centre, Mt Barker West, Hahndorf, Bridgewater, Heathfield/Aldgate/Stirling, ""maybe"" Crafers then express to town, with feeder buses out of each of the stations.
Also, the Seaford Line extended to Victor/Goolwa. New alignment via Aldinga, Willunga then Victor Harbor Road. Trains express Adelaide-Seaford, Adelaide-Brighton, the all stoppers Adelaide-Brighton and Adelaide-Tonsley. Obviously less frequent to Victor.
Of course, without any money none of that will ever happen. If regional rail ever is investigated/trialled, one or two of the platforms at ARS should be dual guage, instead of the Keswick terminal.
Oh, and here's 2c