Depends on how long of day / what day of the week you want -- it is farther out, and no weekend service, but on weekdays there is always plenty of space at the Monocacy MARC station (passenger train) in Frederick MD. You could catch the last direct morning train on the Brunswick line heading down into DC at just after 7AM. Take it to Union Station or you could transfer over to the Metro (subway) at Shady Grove to go to other destinations.
Tickets are simple electronic kiosks at the station.
I rode home the other day with a big spring break vacationing familly group who spent the day at the National Zoo and where staying in Martinsburg WV.
Oh - yeah - Monocacy Station is a free lot.