I should clarify, the proposal for an underground CBD loop in Adelaide is not to be a 'loop' in the true meaning of the word in rail terminology. The train networks of Sydney and Melbourne have shared loops, where multiple trains come in from Point X, Y, Z into the one CBD loop, stopping at multiple stations throughout and then eventually doing a full circuit and and going back out to their respective points of origin.
Once a critical mass has been reached in a train loop system, it actually ends up impeding the efficiency of the train network and causes chronic congestion as due to over capacity and signalling issues, (think of it as like 50 cars on a 200m stretch of a 3 lane road having to merge into a 1 lane road) - thus, meaning that not as many trains can pass through the loop in a given hour. When you've got busy systems like Melbourne, where some lines are already at maximum capacity, having to share the loop with other lines which are also near or at capacity, you begin to understand the magnitude of the congestion issues over there. Fortunately, Melbourne Metro in conjunction with Public Transport Victoria are working collabroatively on a scheme, which by 2030 will actually untangle the CBD loop, and introduce a lot more Point A to Point B services via a couple of CBD stations (not all) throughfares, which is an internationally proven and vastly more efficient way of running a train network.
In Adelaide's case, the idea of a 'loop' really comes from the vision that if a underground link/subway were to be built, on a map, it would
look like a loop, but not actually be one - it will still be a throughfare (as it should be!) taking passengers from Point A to Point B via the CBD. You can see for yourself on page 45 of the Adelaide City Council's Smart Move Strategy envisions an underground train route lining the Gawler line via Adelaide Railway Station > Hindmarsh Square > Victoria Square > West Terrace and then connect to the Seaford Line.
http://www.adelaidecitycouncil.com/asse ... gy-web.pdf
Personally, I would prefer a more route akin to the MATS 1968 proposal, minus the Rundle Mall stop, and City South stop relocated adjacent the South Terrace tram stop.
Any views and opinions expressed are of my own, and do not reflect the views or opinions of any organisation of which I have an affiliation with.