A cable tramway is one option for an aerial system. The load-bearing capacity of a 15mm steel cable is impressively high, and it can be strung along pylons.
Not that you'd want to bolt street cars to such a system, but it's an example of a possible people-mover that doesn't require persuading a multiple-kilometers-long bolus of solid granite it'd prefer being elsewhere.
Whilst I'm aware that Musk's premise here is that he can be the boringest guy ever, I'll note that the Gotthard Base Tunnel, in Switzerland, required 17 years of excavation to extend 35.5 km.
https://en.m.wikipedia.org/wiki/Gotthard_Base_Tunnel
Tunnel systems can also give rise to some interesting cascade failure modes. Cab-forward deisel locomotives aren't generally considered a risk factor, until they are. Summary of an accident report, here, five souls lost to bad design of stock and tunnel, inadequate maintenance, poor judgement, and panic response: