You said
From just south of Agincourt? I will note, I read your text and did not consult your video initially, which now having done so adds no material detail on your statements,. Let's look at line you drew:
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The onus is on you to prove your statement is feasible or you ought not to have made it.
You love to make emphatic statements without providing the supporting evidence.
Let's review your imaginary alignment. There is clearly no space to elevate immediately south of Agincourt GO Station, as you need to traverse the CP Mainline, and you do not have time to make the requisite climb. Even a steep gradient would not get you to the surface in time, never mind clearance.
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Note that I made the favourable assumption that the train was fully turned south, with its north end abutting Sheppard.
Once you're underground under the CP Mainline, you also have to stay under Highland Creek. Note then that if you brought the track up above grade between the creek and the 401, it would only feasible if you relatively shallow to begin with. But, there's a small problem............. You're going to emerge in the midst of the high-risk floodzone for the creek, which would make the tunnels vulnerable to water infiltration.
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Blue is the flood risk from the TRCA flood risk map.
Assuming you avoid this by staying underground until you clear the 401......well, you'll note that the blue continues to the south.
I suppose you could surface in the middle of Progress Avenue and go elevated from there, but that will require considerable expropriation to widen Progress for a portal.
You would then have to rigidly follow the road alignment all the way to the proposed SSE station, and I'm not all clear how that's going to work, including turning radii with the new at-grade crossing of McCowan and the new road grid in the area.
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As a side note, having now listened to your video..............The Sheppard line will be six-car trains not 4. That decision has already been taken internally for planning purposes.
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Once again, please note, I am not arguing against your proposition of going to STC, merely suggesting additional information, which I outlined in previous posts is required to determine if that is the best course (it may be, that is TBD).
What I am arguing is that the case has been made on scant evidence, and that in the course of defending your choice you've made statements which also lack proper support.
Its ok to say "I wish it were this way, or maybe we could do this" but you tend to say "we can and its easy' without either being well founded.