[...] As for the Tonsley line, the best thing to do would be to extend it alongside the Southern Expressway (similar to what they did in Perth) to Reynella, than alongside the Reynella Bypass section of South Road (crossing South Road and the Field River at the same point) and taking over the route...

O-bahns are a specialist transport mode. They are good for connecting points of high demand to large tracts of sprawl, through sprawl. However, if there are any existing railways, it's usually best to go with rail instead. Competing with roads is more difficult - there either has to be a good reason...