>too far apart to reasonably take trains between them //
It's way nicer IMO doing inter city travel by train. One of my favourite journeys ever was from Wales, via England to South France.
Aren't railways one of the main ways to do long journeys between cities?
For any other route, a family would fly and then rent a car if upper class or drive if middle/lower class.
Amtrak sometimes serves these routes, but costs more than flying and takes days instead of hours (3 days Chicago to SF, 24 hours Chicago to New York or New Orleans, and if Chicago isn't on the way, you'll need to detour through Union Station). The max flight time here would be ~6 hours.