The main obstacle so far for commuter rail has been that CN controls the existing ROW and isn't willing to let the city use it for transit, because it's also the only rail access to the container terminals, so they prioritize freight as much as they can. There was some kind of joint proposal with VIA a few years ago but it fizzled out. The city and province both seem allergic to the idea of laying any track of their own. Halifax Transit in particular seems reluctant to introduce "from scratch" rail service, which would mean having to build new storage and maintenance facilities for the trains, and finding rail-adjacent land for those.
If nothing can be worked out with CN over the mid-term then I think an ambitious expansion of the ferry network, with further improvements to the bus network, is what we should aim for in practice in terms of regional transit. A lot of arterials actually have bus lanes now, which tends to work well. Larger-scale BRT could be worked into corridors like Dunbrack-Kearney Lake and various freeways (especially the 102 and 111). One of the problems with this, though, is that there are already staffing shortages for both bus and ferry operators, and large-scale expansion using these technologies makes us even more vulnerable to that.
A more ambitious solution would be a fully grade-separated system (underground on the Peninsula, above-ground off-Peninsula) that can use "driverless" vehicles like the SkyTrain. I honestly think that if the City committed to going that route it would work well - it would not result in net savings for Transit compared to not building it but it would be the most effective relief valve for commuters along the 102 corridor, even if it only went as far as Bedford. As that corridor becomes even more congested it's not hard to imagine a choice between a 60+ minute drive in gridlock from Larry Uteck with nowhere to park once you get to your destination vs. a 10-minute train ride + ~10-20 minutes to get to the station. We're very rapidly approaching the point where it can be pitched in that way and I think the public would support something like that - as long as there's something in it for Dartmouth
I think we could get by with 3 stations on the Peninsula - Downtown (ideally connecting the ferry terminal with Scotia Square), Robie/Quinpool (Commons), and Mumford - and complement these with a network of aerial trams above Robie, with stations at Young St, Quinpool, South St, and then crossing the Northwest Arm with a station at Fleming Park and a terminus at a "Mainland South Park & Ride". The rail line could emerge from underground with a station at Bayers/Joseph Howe/Dutch Village and then just run parallel to the 102 out to Bedford, with stations at Lacewood/102, Larry Uteck/102, and Hammonds Plains/102. Eventually this could extend to the airport or even to Hants County (if they decide to densify enough for this kind of thing, which seems likely) and could include stops in Fall River (if they decide to densify enough for this kind of thing, which does not seem likely) and Sunnyside and/or Cobequid. There is also room for multiple Bedford/Rockingham ferry routes in this scenario since they would be drawing commuters towards the harbour and away from the 102, the opposite of the LRT route, which would be good for traffic flow.
For political reasons we would need to see something on the Dartmouth side, and a system like that could run from Woodside above/parallel the 111, with stations at Portland, Main, MicMac Mall, Burnside/Highfield, and Windmill, then continue above Windmill and Wyse Roads to Bridge Terminal, then cross the Dartmouth Commons (either under or over-ground) to end at Alderney Landing. This could be complemented by an aerial tram running from Alderney along the Shubie Canal and Lake Banook to MicMac or Main, which could eventually be extended along Lake MicMac to Shubie Park and Dartmouth Crossing. Plus a ferry from the existing Woodside Terminal to Eastern Passage. The two systems could eventually be linked, but I think it might actually be an easier sell if they weren't, at first, other than integrating the ferry stations.