Earlier quoted context omitted.
Density over some wide area is the wrong metric here. An entire state is absurdly large (in most cases). Two dense endpoints can be separated by quite a distance and still be worth connecting.
CA is a third as dense as Japan, as whole. Tokyo has 8.9M people while LA has 3.9M people; half the number. Osaka (3rd largest) is 2.7M people while San Jose (3rd largest) is just under 1M people; again, half the number. The distance from Tokyo to Osaka is 500km, while from LA to San Jose is 550km. (In both cases, the 2nd largest city is nearby the largest.) You have half the justification in CA you do in Japan, look…
The US historically had much more passenger rail than it currently does. You haven't provided any convincing reasons why that shouldn no longer be workable in the modern day.