Adelaide, is Australia's 5th largest city in terms of population. Yet we're the only city with more than a million people that has not built underground rail.
Once they're all electrified, trains should go underground into the CBD, Hindmarsh square would be a good start.
Hindmarsh Square would be the most successful because of the following reasons: close to Rundle Mall, Rundle Street (shopping/entertainment), the east end market place, the aboriginal art gallery (i forget the name of), the Adelaide CBD, University of Adelaide/SA-city east campus, Botanic Gardens, O'bahn buses and all east-west bus routes.
But i think ultimatly the goal should be, underground rail at all of the city squares by 2050. With major transfer stations at the existing North tce station, a new South Tce station and a redeveloped Keswick interstate station.
There should be some kind of action on this by the 2020s at the latest. Even though it's likely to cost billions, i think the cost of not doing it would be far greater.