People will still travel long distances, and because American passenger trains aren't of very high quality, they will be driving. The gallons per seat per mile of airliners are way better than basically any car, for example the 747 has 91 Passenger-miles/US gallon. Even some of the better gas cars struggle to get above 30 miles per gallon.

Additionally, its not really a massive win. Air traffic only makes up 8% of less than 30% of American emissions. Cars, on the other hand, make up more than 75% of that same 30%[0]. Switching to aircraft might actually make more sense for the environment.

[0] https://www.epa.gov/greenvehicles/fast-facts-transportation-...