“Then there is the argument why should the rest of the country pay for something that will only be used in the coasts or densely populated areas?”

This isn’t a real question. You’re making up a reason that you haven’t proven exists.

There is no funded HSR outside of California.

What part of it is not a "real" question? Are you saying there are assumptions in the question? Of course, the entire argument is hypothetical since currently there isn't a train system as described above in the US. The point of the comment was to seek predictive reasons in favor and against a hypothetical scenario. This has nothing to do with existing funding inside or outside CA. You seem confused.