In the case of Musk's original proposal posed against HSR, largely because it had far fewer stations and because it had termini nowhere near the notional urban endpoint destinations, because getting close to them required high-priced right of way.

It basically was a way to get from the distant (in transit time from SF) fringes of the Bay Area to the, IIRC, even more distant (in transit time from LA) fringes of the LA Basin.

It seems to me that hyperloop is high speed rail with the addition of a vacuum, which gets you higher speed. The additional requirement of a giant vacuum tube should make the system cost more, not less.

Personally, I’m rooting for Elon Musk to figure out how to make tunnels cheaply with the Boring Company, so we can take that tech and use it to build a bunch of subways. Traffic within urban areas and suburbs is a much bigger societal issue than intercity travel anyway. Subways are a proven solution, but somehow we lost the ability to dig tunnels economically so we stopped building them.

A large amount of cost is in the stations.

In the us, a large amount of cost is inefficiency, projects cost much more than elsewhere, even considering purchasing power parities. Alon levy has written and talked about it extensively, e.g. https://pedestrianobservations.com/2019/03/03/why-american-c...

I don't know how as-actually-built hyperloop compares to rail tunnels.

This is what Tesla did by starting with the Tesla Roadster then Model S then Model 3... and it's what the Hyperloop aims to do. Launch as a small throughput luxury product and if it's profitable then it doesn't matter if it ever expands beyond that or not.

It is very much not.

Tunnels are used only when absolutely necessary to deal with terrain.