One example - I can get a direct flight from my city in Australia to LA, but not to SF. Would be useful to be able to take the train from the airport at LA into somewhere in SF.
So it could potentially replace a lot of short-haul connecting flights.
You are mistaken. I doubt you will choose to take a train from LA to SF if you are already at the airport. I haven't kept up with the details of just how slow "high speed" rail is supposed to be, but to those who live in California just seeing Fresno on the route triggers an immediate reaction of "why are you going 100 miles off the direct route?"
Pulling up a map of Australia, this is the equivalent of setting out to improve transport from Canberra to Melbourne, and the first sections to be built will connect Leeton to Echuca.
So it could potentially replace a lot of short-haul connecting flights.