I agree with Yan. (By the way, I saw about 10 Downtubes on the bike path today on our 45 mile ride.)

Chatrider, the only point of having a folding 26" bike in New York City is that you might be better able to fit it in a cranny in your small apartment. It won't be any easier to take the bike into the subway folded than unfolded. Unfolded, full size bikes are allowed on all subway trains, but not Metro North. That said, the Jack is a great urban utility bike, whether or not you fold it.

I regularly ride on trails with my 18" wheeled Birdy, and have never had a problem. But I wouldn't tackle anything rocky, anything with deep ruts, or any drop offs.