If you look at the various proposals, you’ll see they start like that. You start with focus areas where cities are close together, such as connecting cities in the Midwest to Chicago. You have similar opportunities in southeast, Texas, California, northwest, and of course the northeast where we already have Acela.
However, once those are established, neighboring cities naturally want to be extended to. You can easily imagine that process eventually turning into a connected map - except maybe Great Plains and Rockies