I'm going to focus on your commute rather than the particulars of what bicycle. Let's see, Light rail to Penn Station or Camden station; MARC train to DC; and, metro. Most of the time on the MARC you are going to have to stow your bicycle in the vestibules on the single level cars or in the few luggage compartments in the ends of the double-decker cars. I have seen commuters with 16" and 20" Dahons, Bromptons, and Stridas on the MARC trains. Only the Stridas fit in the overhead luggage rack on MARC trains. I've also seen a commuter use a folding razor scooter (foot-powered) for his commute from the MARC train in Baltimore.

Remember, if you are riding the Penn Line, it is usually packed -many times S.R.O.- coming home to Baltimore. If you are going to buy a bicycle, be sure to take that into consideration. Good luck with your commute.