I'm not sure if this would work, but I was thinking if you bought the Ontario Southland railway between Woodstock and St. Thomas, then you could re-build the St. Thomas to Glencoe railway. You could then pay CP to swap tracks with you. That would give you a route between Glencoe and Woodstock via London for HSR.
Between Woodstock and Paris, you could either negotiate with CN, or more likely, expropriate land next to CN's right of way to widen it. It's about 30 km, mostly through farmland, but by following CN's ROW, you'd avoid bisecting parcels. That would make expropriation easier.
Paris to Lynden is an abandoned railway with parts of the land sold off, but perhaps it could be rebuilt.
Lynden to Copetown is 8 km of active CN track. Two options there. First, you could widen the ROW through expropriation. Alternatively, you could re-build the Copetown to Brantford rail line and pay CN to swap their existing line with you (which would also give GO trains a route into Brantford).
From Copetown, you could follow the abandoned route into Hamilton to CP's Aberdeen yard.
From Aberdeen yard to Aldershot, I think there are a couple options. Either you negotiate access with CP or, less ideally, you skirt Aberdeen Yard and tunnel 2 km under Dundurn street and York Blvd towards the lake. Then fold any widening for HSR into the junction work GO already needs to do (though part of me wonders if the junction could be better fixed by rebuilding the CN line between Caledonia and Welland).
Total route between Union and London via Hamilton: 175.8 km, most of it pretty straight, compared to 190.0 km via Kitchener. Kitchener would probably be easier to build, but the southern route would better serve people in Hamilton, Niagara and along the lakeshore.
