"Metro North" has complicated rules, basically if the conductor wants your bike off the train they can order it off. If you think you will be on the train when it is crowded I would bring a drop cloth, or even large garbage bag to wrap it in, turning it into a package. In the mornings PATH requires you to fold, and they have cops at the major stations to make sure you do.

As to trails the issue is really the tire width. Folders tend to have narrower widths that do not work on loose or muddy surfaces that allow the bike to sink. On hard packed trails you will also have a rougher ride due to the small dia wheels.

The full size folders mostly allow you to put them in the trunk of a car, which seems better than the roof, the cargo bay of a bus. They are also useful if you are doing a day trip on a commuter railroad.