If the Belair and the Port Adelaide group of lines were converted to light rail connected to the Glenelg - Hindmarsh line then they would run through the CBD.
This leaves the Seaford and Gawler lines: bring them into the city under the length of Currie and Grenfell Streets terminating in a balloon loop in the east park lands.
This would be constructed by "cut and cover" with two platform stations at Morphett Street, King William Street to Gawler Place, Frome Street to East Terrace, and a single track station, Kent Town, under the Adelaide Bowling Club.
While the trench is being dug it could be deeper between King William and Pulteney Streets to accomodate a Queens Street Brisbane style bus interchange free of any other road traffic.
Lots of dollars, though - several train loads in fact.
Now we are talking!