I also still can't quite believe the very ambitious start schedule - there is, there are, a lot of complicated infrastructural elements in the path of the tunnel that will need to be accurately mapped and permits to be issued before he can start - I just can't see them having assembled all of that as quickly as claimed.
What is the propsed path?
I don't think it has been determined, although I imagine under or near the Kennedy Expressway would be the most direct route. Quite frankly, if this gets built, I will be quite surprised. It just seem like pie in the sky. Musk seems to be promising everything to everybody.
I keep (well, kept, haven't checked up lately) hearing that it'll run under streets and existing rights of way, so little to no legalities of being under private property. I really don't know much about the law for that here - we have the deep tunnel system for drainage here, I think it runs under private property in places. The cta subway sections here don't run under buildings much, if at all, nor does the old freight tunnel system.
There's still the issue of avoiding underground utilities, though if it's deep enough not an issue, but there is still, you know, geology to deal with, borings and core samples to take. And of course, it'll have to avoid the deep tunnel and you'll need access points to remove tailings, emergency access/egress, etc, on the surface. Depending upon how deep they go it could be through bedrock or through clay if shallower (I'm assuming clay because that's what seems to get removed at high rise construction around here when the drill for caissons or footings). I know the deep tunnel system is bored through bedrock, which around here is limestone.