I don't think Musk want hyperloop to be built above the ground. This is just my own personal opinion, not sure if that's how it's going to work, but in one of his interview (TED? I think), he mentioned one of the reasons for his Boring company, was it would be relatively cheaper to build hyperloop - because the atmospheric pressure will be lower underground - and since you don't have to worry about building large infrastructure for only one purpose (hyperloop) - the cost will be much much cheaper. Because under ground tunnels can be build for both hyperloop and roads for cars and it scale better than above ground.

I don't think above ground is economically and technically feasible, there are just too many unanswered issue that almost no one has answer to. Not to mention it will be very very expensive and time consuming.