The 34th st. station and the Hoboken station have elevators. World trade center has a series of stepped elevators, which take like 15 minutes to get up. Christopher St. doesn't have an elevator. I don't know about the rest.

I'd have to agree that the morning rush looks like a nightmare for a folder, but I don't see how it would be any worse than a large duffel if you had a completely closed Dahon bag and saved the folding/unfolding for outside the station. I've never tried it, though.