Would it? We've been using basically the same electric subway technology for more than 120 years already. We might have something different in another 100 years, or we might not. Note that if anything, the pace of progress in mechanical arts is slower now than it was in the late 19th century.

What good is hyperloop for cross-city travel?

People in 1915 were I am sure imagining a future of personal flying machines which they were confident would be in operation by 2015.

Here we are, still cramming into the same tubes that people were travelling in (albeit probably less crammed) over 150 years ago.

Hyperlopp is very very low density comapred to a tube network. It would destroy the capacity