Not sure whether I would go with cut and cover, but the route is how I was thinking.
Given the soil conditions in "Sandy Hill", cut and cover would be the safest option to avoid another sinkhole. Remember, U Ottawa station was initially proposed to be underground, and they shifted the portal back to just south of Laurier due primarily to soil conditions.
Please leave the SE Transitway alone. It is more useful as a BRT than it ever will be as LRT, and it is unlikely that its capacity will ever reach a point where it needs to be upgraded beyond BRT.
If Bayview really becomes an issue in the future, that's when we start talking about Bank Street. No need to start cannibalizing other corridors with half-measures.
VIA can get in on the action too with a Billings Bridge station.
Once the Trillium Line starts carrying the bulk of the South Keys area ridership, the SE Transitway will be a bit of an orphan. Yes, it will still have utility as the defacto extension of the Baseline BRT, but there are other options for that corridor for sure.
For what it's worth, I still think redirecting and electrifying the Trillium Line under Bank St is a better option than routing to Union, but the Union option should at least be given a preliminary look before being discarded. And FWIW, the alignment I prefer is current alignment to Carleton, then under Colonel By to Bank, then up Bank. That way you still maintain access to Carleton.
The problem with this is that the primary destination of Line 2 is Carleton University, and this redirection of Line 2 would completely avoid it. This is why the conversation was redirected towards the reactivation of the Beechburg sub, and whilst I did use the term "Commuter Rail", It'd be more like another DMU service similar to the Trillium Line, except this one would run from Northern Kanata to Tremblay (or possibly the Union Station). Additionally, there is some logic to maybe extend Line 4, and have that go up to Union Station, instead of terminating and forcing a transfer at South Keys. The issue of course with all of these options is at what point is there enough ridership to justify reactivating Union? Ideally, you'd probably want more than just the Beechburg Sub line, and the airport line. Maybe a local line travelling to Smith's Falls, or maybe a line that serves some potential future developments out east along the VIA Rail corridor. Only once you have a substantial network of lines that could radiate from the city center, would building the Union spur actually make sense.
That's why I proposed Line 5, which would use the Line 2 corridor north of Ellwood Diamond. Line 2 customers would be able to transfer to Line 5 at Billings. This routing would also have the secondary benefit of connecting Carleton directly to Billings.
This is why the conversation was redirected towards the reactivation of the Beechburg sub, and whilst I did use the term "Commuter Rail", It'd be more like another DMU service similar to the Trillium Line, except this one would run from Northern Kanata to Tremblay (or possibly the Union Station).
Yes, I see the Beachburg sub as being quite valuable as an original Trillium Line-like corridor, a true hybrid between LRT and regional rail.
Additionally, there is some logic to maybe extend Line 4, and have that go up to Union Station, instead of terminating and forcing a transfer at South Keys. The issue of course with all of these options is at what point is there enough ridership to justify reactivating Union? Ideally, you'd probably want more than just the Beechburg Sub line, and the airport line. Maybe a local line travelling to Smith's Falls, or maybe a line that serves some potential future developments out east along the VIA Rail corridor. Only once you have a substantial network of lines that could radiate from the city center, would building the Union spur actually make sense.
My preferred overall phasing would be this:
1) Complete Phase 3 of the existing plan.
2) Convert the SE Transitway to rail (flexible enough configuration to be used by DMUs, VIA, or whatever else). Add Line 5 (the U line) and extend Line 4 to terminate at Hurdman. This would at least avoid having to make 2 transfers to get from the Airport to downtown. This configuration would mean that you would have Lines 2 and 5 terminating at Bayview, and Lines 4 and 5 terminating at Hurdman. Line 5 would also provide a downtown bypass option.
3) Add Line 6, which would be a Beachburg Sub line that would run from Hurdman, via the SE Transitway corridor to Ellwood Diamond, then hop on the Beachburg Sub through Nepean (transfer to Line 1 at Knoxdale Station) and up to the north end of Kanata or to Carp.
4) Examine options for a Bank St Subway, or extending all services that would end at Hurdman into Union station instead. If the Bank St Subway option is chosen, use the alignment I proposed above (Carleton/Colonel By/Bank) and have the future Carling LRT join that corridor around Dow's Lake.
The Trillium Line's capacity is not that high. If we were to do an "Ottawa Relief Line", a Bank St subway would be a better choice. A conversion + tunnel + new station wouldn't be cheap. Why spend so much money on half-measures?
As others have mentioned, its capacity is limited by single track sections, particularly the Rideau River bridge and the Dow's Lake tunnel. Upgrade those two infrastructure items and you could have the line running subway-level frequencies.
And with a Union option, I don't know if you'd need to convert the whole line to electric. You could go with some sort of battery hybrid option that would work for the relatively short tunnel stretch.
And neither option would be cheap, I'm just saying study the alternatives instead of assuming that Bank St Subway is the best way to go.