There are areas in the US that DO have the population density to support mass transit but they aren't nearly as large as the areas of Europe that have that density. Even a state like New York with NYC, Albany, Buffalo, and Syracuse also has a massive upstate area that is pretty sparsely populated. California is another great example. There are a ton of people crammed in between the mountains and the Pacific but the rest of the state is mostly empty desert.
There's a reason that Washington-NYC and NYC-Boston are the two functional rail lines, and why LA-SF won't be, even if it's HSR.
Both of those East Coast corridors will get you city to city in under 4 hours. HSR in CA is estimating 6 hours. I honestly believe about that 4 hour range is the upper bound beyond which rail travel fails.
I highlighted earlier that flight time from SoCal to NorCal is 1 hour. But obviously that's not how long the trip takes. I need to budget 30 minutes to drive to Orange County to park. I prefer to get to the airport 60-90 minutes before flight time to get checked in (if traveling with my wife, that includes checking backs which I don't do when I travel alone), get through TSA, all so we can start boarding 30 minutes prior to takeoff and not be stressed over time. Then it's an hour flight. Then 20-30 minutes to deplane and get those checked bags. Finally, then there's getting a rental car, or if we're going to SF, taking a 30 minute BART ride up into the city.
So that's a 4 hour trip, and that's assuming the flight is on time. By flying I've added 3 hours to my actual trip outside transit time.
Anything near or less than that, and I honestly would rather drive. I.e. my wife was talking about flying to Vegas when we go in Sept, and that sounds like a hell of a lot of a hassle to avoid a 4 hour drive. Since we're going on a Saturday morning and coming back first thing Sunday morning, we'll avoid the only thing that usually makes me willing to fly--the Friday afternoon HELL trying to get out of Orange County on CA-91. Even for going up to the Bay Area, it's a 5-6 hour drive but if it's something where having a car is useful (i.e. Napa/Sonoma), I might even choose driving anyway. If it's downtown SF where I wouldn't dare have a car, then flying obviously makes sense.
Now compare it to HSR. The advantage to HSR is that you don't have the TSA, you don't check bags, you don't have checkin that you have to arrive 90 minutes before the train leaves, etc. Literally you can show up 10 minutes before the train leaves and hop right on. And when you arrive at your destination, you just get off the train with your bags and go on your way.
So for me, since I don't live near Union Station in LA, I would probably take the 10 minute drive to the Irvine metro station 10 minutes before a train leaves, there are frequent trains that get me up to LA in an hour, and as long as I don't cut it SUPER close I might wait 20-30 minutes for the HSR to leave. And then it's 6 hours and drops me off in downtown SF (I believe). So yeah, it probably adds about 90-120 minutes total, but considering I have to make it from Irvine to LA, that is actually an hour of real transit.
You could say I'm cherry-picking, because if I had to fly out of LAX (60 minutes drive plus factoring in 30 more minutes for traffic) for example it would be a lot different than flying out of Orange County. But I'd argue that since there are 5 pretty significant airports in the LA megalopolis, pretty much everyone lives within 30 minutes of an airport.
But it's 4ish hours by plane, 6ish hours by car, and 8ish hours by HSR. If HSR were cheap, it would make tons of sense to not have to worry about driving. But if HSR is as expensive (or moreso) than flying, there's no way it makes sense to double my trip time and pay anywhere near the same cost.
