We took the VRE from Fredericksburg, Virginia into DC. They also go from Mannasas (and all points in between). We parked at the station the first day (dummy us) because you have to have a pass to park there (as with ALL OF DOWNTOWN FREDERICKSBURG for more than 2 hours). You had to be a citizen of Fredericksburg to park there. Both of us came back to a ticket on our tanks ($15 or something like that).
The next day, we paid the grocery store across the street $10 or something and he gave us a thing to hang on our bikes and we got to park there all day.
I think the train ticket was $10 round trip and was a little over an hour. Since we hadn't been on a train since we were kids, even the train trip was cool. Plus Fredericksburg has the battlefield (Gods and Generals) and Spotsylvania, the house where Stonewall Jackson died and Chancellorsville (where Stonewall Jackson's arm is buried where he was shot at) are all right there as well.
The only thing about the VRE is that it is a commuter train and I think the last one leaves Washington at 7:00 pm or something, so we never got to see DC at night.
Also, the hotels are more at Manassas but the train ride is shorter.
Here is the link to the VRE with schedules and fares.....
http://www.vre.org/