Depending on where someone can pick her up, the train station to use might be BWI (the Baltimore-Washington Airport) or New Carrollton (where US 50 and the I-495 Capital Beltway cross). It's also possible that someone can pick her up at Union Station in downtown Washington, DC (within a couple blocks of the US Capitol) or at Penn Station in Baltimore. Many NorthEast Corridor trains stop at all of these stations.
If Marion gets a ride, many folks from the DC will be passing close to New Carrollton, while anyone flying in via Southwest Airlines will be at BWI.
Transport to West River from a train station involves either carpooling with another Getaway person or a very long taxi ride (though I suppose the airport shuttle van from BWI might also transport someone from the rail station).