Which might be considerable on an old Victorian railway in the light of difficulties experienced by many main line wiring projects.If what you say is correct, it sounds like costs will not be much lower from using tram trains compared to conventional HR. The only saving will be 750 v DC electrification compared to 25 kv AC.
Additional costs for making the tram-train vehicles 'street legal' are fairly low and mainly revolve around providing brake and turn signals, side skirts and fenders, extra powerful brakes (usually magnetic) and improved all round visibility from the drivers cab.
Good extension ideas for the future there, all possible with tram-train technology. I also like the interchange idea for Callaghan Square. Perhaps linked to the redeveloped south entrance at Central via a covered pedestrian walkway through new developments on the south side of the square.If it's going to be light rail to the Bay, why not use this added flexibility to have another stop around Callaghan Square for easier transfer from non-Queen Street services at Central (and better access to the metro from the southern part of the city centre, without having to walk to Queen Street) ? And what about an eventual extension of the line from the Bay over that causeway thing to Penarth ?