Considering the density of development along the Port corridor and the usefulness of the present CBD as a transport hub one has to wonder if heavy rail is still the right answer for the Outer Harbour line.
I well recognise the slow transit time from Hindmarsh to North Terrace by the present tram does not stack well against heavy rail from Bowden. However the tram time could be improved with priority at traffic lights. The tram serves much more of the CBD than the train which is stuck to a terminus appropriate to the 1856 steam age.
Back in the 1930s the Glenelg train was replaced by a then modern interurban light rail system and in comparative terms is even more viable now than it was in 1930.
In overseas cities I have visited, what have obviously been steam railways have been converted to light rail and are fast and popular using mix of street rail (with the local trams) and dedicated (old steam) ROW. Often these systems are in direct competition with the heavy rail and still flourish.
The features of the vehicles are, electric traction generally low voltage dc, street level entry, multiple unit capability, all the communications mod cons, a good turn of speed on the open track.
All this is to say that taking the Port Line as heavy rail under the SG line is a waste of money. Converting it to light rail with vehicles I have described from OS is better value.
Initially the line can be diverted onto the present Port Road alignment at Bowden. As traffic, both road and rail demands, and money becomes available a new alignment off Port Road can be provided. The new alignment could go up Currie street and then continue along a convert O'Bahn.