Reply #8: you cannot take MARC on the weekends

[northzax]

which leaves you two options from BWI for public transport to DC. You can take Amtrak, which does run, (an Acela wil cost about $25, an unreserved regular train $13, last time I did it) or you can take a DC City Bus (leaves at the far right of the terminal) for $2.60 to Greenbelt, and the Metro in from Greenbelt for another $2 or so. That bus runs every hour and takes about 45 minutes. Add in the Metro and you're looking at 1:15 tops from/to Union Station. (for the way out, make sure you check www.wmata.com to find out when the bus you want leaves Greenbelt, you don't want to wait for an hour at a metro stop, ain't nothing to do.

check out www.amtrak.com for train schedules from BWI rail station to Union Station in DC.