Couldn't a pushbox, like that used for HuLRT at Lakeshore Line be used to get a cut and cover line under the Stouffville line? Seems like launching a TBM would be considerably more complex.
I don't believe so....... let me show you why:
In the above, lets assume your subway/metro is approaching from the west towards the Stouffville Line
Notice that immediately east of Kennedy is Highland Creek. If you are going under the GO Line you do not have room available to get down to the right grade for a push box, post-Highland Creek.
If you're going under Highland Creek, you're not going under cut-and-cover. I mean, in theory, its not utterly impossible, but you would have to coffer dam the creek and divert the flow........in practice, I don't see that happening.
Additionally, assuming you're continuing east in the Sheppard Corridor, you have the CP mainline only ~600M to the east.
Going up and then coming down again in a very short span is an issue.
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Its not inconceivable to go elevated through the entire section, but because of the creek and the 2 railways, you have to stay constantly elevated, at a height that allows for double-stack freight/GO and catenary underneath, The challenge of doing that, and finding room for a station, and feathering that into a connection to the existing GO Station as well..........
I just think going under is the less complex and costly route, in that section.