I don’t have any info on folding bikes, but you could consider Capital Bikeshare as an option. For a year I rode my bike to the VRE station in Burke Centre VA, took the commuter rail to L’Enfant or Union Station, and then picked up a bike at the station and dropped off near my office. It kept me from having to invest in a folding bike, and to worry about carrying it on and off. May not work for you but thought I would suggest it.
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