I think the way to think about it is that 25kV AC via overhead lines is the way to go for mainline railways, but for shorter underground segments using potentially smaller trainsets, third rail DC will be cheaper and simpler and more than adequate. They are also the de facto international standard for metros, so there is a large availability of trains designed for this configuration off the shelf.
And by the way, I would be advocating for a smaller trainset with a highly frequent service. As well as delivering shorter wait times, it spreads the loads out at stations, enabling simpler smaller stations designs. I’m envisaging simple portals at street level connecting to two or three escalators and a lift going straight to a single island platform.
What about also doing, Spring Hill to James St to Kangaroo Pt to the ‘Gabba to South Bank to “Station X” in the Inner West (Red Hill/Paddington perhaps) to Spring Hill - you’ve piggy-backed off your existing model, to also deliver a 360 inner-city circular loop to drive strategic asset invetment value in developing the broader Metro proposal and you’re keeping 1,000’s of people a day out of the congested inner-CBD!
Are river crossings (as part of TBM tunnels) more expensive than through earth? There is an unavoidable problem where the subway tunnel would benefit from lots of river crossings, especially when it is close to the city which is built around the river.
I don’t know that the river crossings themselves are more expensive, but crossing the river means the tunnel needs to be substantially deeper, so any stations nearby are substantially deeper and therefore more expensive.
Generally they are significantly more expensive per metre, especially as you go downstream where the channel widens and alluvium increases in depth and extent. A river channel will usually be comprised of weak alluvium to significant depth which is very difficult to tunnel through. You could use earth pressure balance or slurry tunnel boring machines but that would still usually require sinking the tunnel at least 15 m below the riverbed. In a hilly city like Brisbane that means stations very far below ground level near the river, which means more excavation which is a significant cost. There are other options like using an immersed tube which is laid down on the bottom of a river from the top and then buried. The main problem for the Brisbane river is that it is heavily trafficked and relatively shallow, so any construction from above the riverbed would be extremely disruptive during construction and probably afterwards. It’s by no means impossible but a rail tunnel river crossing downstream of around Bulimba probably wouldn’t represent good value for money.
The Vancouver Skytrain manages quite steep gradients (you can see the track diving after the station), which enables the tunnels to vary in depth quite significantly. In other words, it could be much shallower at the station than it is at the river crossing even if it’s only a couple of hundred metres away.