To me the hyperloop in my head would be a maglev train within a vacuum chamber, unless Musk has some kind of new tech in development in his research lab somewhere (not implausible if you ask me), but if i was going to build such a thing i'd probably have to do a working prototype first and it seems the easiest way to do that would be to use an existing underground train system, perhaps a derelict one. Then it would be much easier to create new infrastructure after its been proven to work.