You folder fans may enjoy the the New York Times story about folding bike users. The article talks about people like Brompton-owner Stacey Bhaerman, who takes her folder on the train to Manhattan. 'Curious strangers along her commute ask her about the bike so often that she is considering carrying a sign with answers to the most popular questions: "Costs $1,000. Weighs 26 pounds. Folds up in no time."'
Most U.S. transit agencies don't have a formal policy on folding bikes because they fold to something not much larger than a stroller. Of those that do have a policy, most allow folding bikes on board, while a handful do not. Denver RTD -- which serves the area I live -- does not permit folding bikes on board buses, though you can put them in the undercarriage luggage space. On trains they are subject to the same rules as any other bicycle.