Is quadtracking the CP mainline through Toronto possible?
In general, the west end of the corridor from The Junction to Summerhill is already more than large enough to support that, yes.
There would be a cost, but subject to CP's agreement its fairly straight forward.
Where you go east of Rosedale there are some limitations (which an can be overcome, but will cost $$$, may spark opposition from some property owners adjacent the ROW.)
This is an overhead image of the corridor at Dovercourt, and you can see the space available in the ROW:
Obiviously, new track woudl be required and some new rail bridges as well, but no property acquisition.
Below is a section just east of Yonge, which might require an embankment widening (though the ROW is large enough).
Hideously expensive though, would be quad-tracking the bridges (meaning a new bridge) over the West Don Valley:
East of this area, there may be costs for retaining walls in some locations (and other bridge works)
This is the ROW east of Victoria Park Avenue:
In summation;
Yes it is do-able, but some portions would be quite expensive; while others would be much easier to implement.
Adding passenger rail here is viable; but as it is CP's mainline, unless one is displacing CP traffic somewhere else, large investments would be required.
Though, universal 4-tracking may not, in fact be required on all segments (though it certainly makes frequent service much easier.)
