Oh there’s no denying it’s doable. But imagine this; a contemporary underground subway drums up significant resistance from locals. Surface rail even moreso- we put things underground because of it. Now imagine that you are buying up quite a few peoples homes (at least hundreds of pissed residents) through the centre of various neighbourhoods (thousands of concerned residents) because you want to “reuse” something that no longer exists- it’s an alignment that doesn’t really hit any destinations. This situation your describing is comparable to when people suggest reactivating other abandoned corridors that HAVENT been built over. If people aren’t open to losing a trail in a valley 50m away from them, then how is this remotely as possible?
Since this is a question of land acquisition, you would have to pay effectively market rate for each house. Let’s go with a baseline assumption that today, a typical Scarborough home costs $1 million (if this seems high, just pretend it includes legal fees). 100 homes is $100M. Google earth puts 10 homes in the adjacent subdivision approximately 100m, so your gonna pay roughly $100M/km in property acquisition alone. Housing crash? Okay, $50M/km. That’s still an absolutely massive cost most projects hardly even have to factor at all. At this point, even with a measly cost to build the actual Line at $50m/Km, your total is going to be $100-$150m/km. That might be a bargain compared to what we pay for tunnelling, but if we want to be ultra cost-efficient then elevated is only marginally more expensive and well within the Overton window by comparison.
Point is, this isn’t underground because it was the best option. Money wasn’t even a consideration. We could come up with a million better ways to do this and it wouldn’t have put this extension one metre shallower.