Hypothetically, how much would it cost to bury the ground level portion of the Crosstown? If signal priority doesn't make a dent in the transit times, would there be a political appetite to bury the Laird to Kennedy portion?
Setting aside the merits or likelihood of such an exercise, lest my answer spur such replies, I'll try to give that a serious answer.
First though we have to set some parameters.
1) I'll work with the assumption that we are retaining the current running concept (train lengths, catenary, low boarding platforms.)
2) I'm only considering the east end.
3) I'm going to look at this in chunks:
Brentcliffe Portal to Don Mills
Don Mills to Wynford
Wynford to Bermondsey
Bermondsey to Kennedy.
There is a logic to this based on how the current surface network is supposed to operate, whether bridges/river crossings are required, and effects on existing operations.
** Note that I am not including any rolling stock costs, maintenance costs or debt servicing costs, though the latter is surely mandatory, but too fuzzy to predict based on how the financing is done.
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Segment 1 (west to east), Brentcliffe to Don Mills:
I could provide an estimate for full undergrounding, but it would be astronomical because you'd have to rebuilding the existing tunnel at least back to Laird Station and you might even have to rebuild the station. Getting the LRT to go under the Don River would mean a very significant re-grade of the line, its also not clear to me that the math would allow it to get back up high enough for the station at Don Mills in time.
So I won't even work out the details.
Instead, I'll give the alternative. You shift the alignment of the LRT so the south side of Eglinton at all times until it reaches the Portal into Don Mills Station.
There is an obvious problem with getting the traffic under/over the LRT at that point, but its do-able and gives full separation from the Leslie Intersection which is the only issue in this segment.
Except for the cross over this would be relatively 'cheap', providing the existing bridge structure can support the shift. I will assume that it can for the purposes of costing.
Estimate: 450M (very back of the envelope, but it should be ballpark accurate)
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Segment 2, Don Mills Station to Wynford.
There are a series of challenges and assumptions you have to make here. You want to maintain grade separation, but you have to decide in advance what you're doing w/the line east of Wynford (how are you crossing the East Don?)
I will decide on our collective behalf, that we're using the same strategy as the previous segment in that we're going to keep the line on the existing bridge. That means we don't want or need to deep-dive.
However, given the existing depth of the line here, I'm not confident we can do this w/o reconstructing the DVP/Eglinton interchange. We might be able to, with cut and cover, but it would be quite disruptive.
I will assume we can get away w/leaving the interchange. But what to do about Wynford itself? The road passes under, but there's a traffic lit connection to Eglinton. What do we do about a Station?
Very cursory look suggests to me that the LRT should stay underground at Wynford, but at almost the exact elevation of Wynford today. That means we need to rebuild, and re-align Wynford to go over the top, at-grade.
Estimate: 600M
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Segment 3, Wynford to Bermondsey
Here we would go underground, likely by boring, immediately east of the river crossing.
Bermondsey would then be a deep station.
Estimate cost: 500M
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Finally, we have Bermondsey to Kennedy, I'm going to assume this is entirely underground, more or less, though it would want to try to get it level with the existing portal into Kennedy Station if feasible to avoid reconstructing that and rebuilding Kennedy.
This would likely entail a closer of Ionview. its also problematic that it would to pass under Taylor-Massey Creek then come back up. One could consider alternatives, but none are particularly appealing or easy.
Estimate: ~3.5km of tunnel + deep stations at Victoria Park, Warden and Birchmount (I'd cut Pharmacy in adittion to the mid-block stops to make this work), 1.5B
All-in, 3.2B with rounding.
* note the items I omitted; in the real world there will be debt to be serviced, that could double the cost. Also I have excluded the cost of removing the surface track and reconstructing Eglinton. That would be at least 200M, possbily double that.
There's a lot of IFs, ands and Buts there.
But I thiink a realistic range would be 5.5B-7.5B inclusive.
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Line would be shut down for 3 years minimum, up to 5.