A 90 minute train ride connecting Texas' largest two cities? Compared to the 4hr trip a car takes or ~2hr for a plane (factoring in security)? At a (tentative) price point of ~two tanks of gas?
I'm optimistic that it won't only be a commercial success, but also that it will convince people (and politicians) that a HSR-network is in the public's best interest, even for routes that don't necessarily make sense from a commercial (private) perspective.
As population grows, train speeds increase, and road traffic worsens, we may need to rethink what we consider a "region."
Alas, that’s a pipe dream, much less bringing New Haven or Philadelphia into the reasonable commuting radius which is what I suspect you are alluding to.
But to be viable, it has to be competitive with airline travel, and that means fast trains. It currently costs more to go from SLC <-> SF than by airplane, and it takes a full day (18 hours or so, I forget exactly) to get there vs 2 hours or so by airplane and 10 hours by car. If it was 4-6 hours (120-200mph), it would be competitive with airlines if you take into account security and baggage on both ends. I know I would take Amtrak if that were the case, but for now, I'm only going to take it as a vacation in itself.
There are a lot of companies in CA that have offices in Utah now, so improving the train system between them may make sense if they can get speeds to be reasonable.
So, you're either blasting a giant gap in the Sierras, blasting a giant tunnel under the Sierras, or have some novel HSR technology that'll run over the Sierras.
For a much less useful route, it'll probably be more expensive than the actual full proposed CA HSR system.
For reference, the LA<->SFO train was last estimated about $75-100 billion to complete before it was canceled. Of course, there's almost no way anyone could build such a train tunnel in the USA for the same price as they did in Switzerland.