Not so fast.
Passenger air travel is becoming ever more fuel efficient. Airlines are keenly interested in the lowest fuel used per passenger seats, especially the low cost airlines. EasyJet's fleet (a low cost European airline) is almost brand new, same with RyanAir (who are notorious for making everything as cheap as possible). Not only do the airlines want efficient planes, but they want them as full as possible. EasyJet's load factor is 90% for example (meaning on average at least 90% of the seats are filled).
EasyJet's A319-neo aircraft have an average fuel burn (no wind) of about 2L/100km per passenger seat (about 115 mpg (US)). Adjusting with a 90% load factor about 103mpg per passenger flown. This is roughly equivalent to a reasonably efficient mid-size car carrying 3 people (note: most cars most of the time only carry 1 person), but remember the plane is doing 500+ mph while getting this efficiency, whereas the car will only be doing about 60mph to get that efficiency per seat.
A well-loaded electric train can better this of course, but airline travel isn't as absurdly fuel thirsty as you presume - there have been very impressive efficiency gains over time.