I know there have been discussions on completing this link (ie Labor's Metronet and Coalition's promise of sometime in the future).
I think it is a good idea as much of the infrastructure is already in place, eg the carriageway, tunnels on the Armadale and Mandurah lines.
The question I have is have any of the costings done to date include what would be a considerable upgrade cost of the level crossings on the Armadale line? The reason I ask is that the level crossings are already substandard (and appear in the top 5 worst suburban crossings) and in need of grade separations.
The other week I was stuck turning right into Wharf Street for nearly 10 minutes as six separate trains used the crossing (a couple of them had an inordinate wait as the crossing was still active before the trains arrived at the Cannington and Queens Park stations). I cannot see how this could be sustainable with extra trains on the missing link.