Is standardisation of the suburban network really that big a priority, or necessary? Standardising the Belair Line isn't going to magically restore double track operation, the ARTC freight line would continue to run on its own line regardless which benefits both freight and passenger trains; freight trains don't need to be timetabled around stopping passenger trains, and a broken down freight train won't stop passenger trains from running. Passenger trains won't return to Bridgewater along the current alignment either, the journey time is just too slow. Same case with Mount Barker.
A better way would be to realign the Adelaide-Wolseley line out of the inner hills. The map below is a hastily made simple version of maps I've already made, but the idealised bypass in pink (tunnel and viaduct combo, obviously won't be a straight line) will run Mitcham to Crafers, then follow SEF from Crafers to Bridgewater (orange line). From there, there's two options:
- Option 1: Rejoin the current alignment (blue) - cheaper than realigning the entire line, but doesn't include Mount Barker on the main line
- Option 2: Continue through to Monarto South (lime green) - more expensive, but faster and more direct route, and includes Mount Barker on the main line
Added bonus is that removing the freight line from the hills means the current SG track from Mitcham to Belair can be converted back to BG for double track Belair Line. Then, duplication only needs to happen between the city and Mitcham instead of the entire length of the line.