As others say, Narita Express and Keisei's Skyliner services are reservations-only. So they'll never get packed.

But if you take Keisei's regular limited express services even from Narita airport departing around 7 a.m., you can certainly and truely experience what it is like to be on a crowded -- mildly put -- train.

These Keisei trains are simply quite empty with only few passengers at Narita airport but it will pick up people and get full after a few stops. They were built as commuter train services. Narita airport happened to be built be at the end of the line that was extended.