Right - public transit needs to be usable in the place you’re traveling to if you’re going to take a train. This is why a lot of people would rather drive from, say San Francisco to Los Angeles. Suppose you were to take a train instead. Then… great?! what would you do next? You wouldn’t have anywhere to go, so you’ll need a car anyway. You’d either have to rent one or just skip the train and do the drive instead.
Probably a lot easier and feasible in my opinion to build the local public transit first, and then focus on the regional/national transit system.
If you’re going from LA to SF you’re fine. You’d take the Coast Starlight to SJ, then you’d transfer to Cal Train, and that drops you off at the Transbay Terminal in SF which gives you easy access to BART or Muni and all of the streetcar and bus lines. Owning a car in SF is more trouble than it’s worth for a lot of people. I never owned one when I lived there.
Granted, SF is one of only a handful of US cities where this is true.
Heading south to LA would probably be a much bigger problem though.