I think you hit the nail on the head. LA-SF is the busiest air route in North America, and it's one of the busiest air routes globally regardless of how it's r

I think you hit the nail on the head. LA-SF is the busiest air route in North America, and it's one of the busiest air routes globally regardless of how it's ranked. On the number of seats flown per month it ranks comparatively low (number 18), meaning there's a bottle-neck being demonstrated. Either driving is too viable an option, or the airports can't handle extra flights. The US is also notorious for having poor passenger rail. Seoul-Jeju carries a lot more passengers, a couple million, but they already have high speed rail, which makes the run in a time comparable to a flight. So being able to shave a half-hour off a commute isn't going to be commercially viable when implementing an entirely new infrastructure and design. Many routes out of Tokyo carry comparable numbers to LA-SF, but again Japan also has high speed rail - I think the time saving on the Tokyo-Okinawa (1500km distance) would be the only one capable of competing against high speed rail, but Japan is highly populated so likely not suited for massive and unproven new infrastructure projects. The next option is in Brazil, is a slightly shorter route, but the issue being the economy is less likely to be able to support such a project, and the risk of corruption and embezzlement there could tank the entire project with cost overruns. There's really no other credible short distance option in North America. New York to Florida (Miami 5.5mil + Orlando 3.7mil) would be a credible option, NY-Miami is a 2,000km run, passes by Orlando at the 1,750km mark, and the ticket price is almost triple that of the LA-SF run. So if LA-SF isn't an option it's "Go Big or Go Home". Crossing states would be a big issue. So we're likely going to see this proven small scale on commuter routes in and out of big cities.