A few more that I’m working on but haven’t ridden all of, or any of, or photographed, or whatever:

• Ted’s Challenge: 100% paved, stop for rosé instead of beer.
• Fort Circle Hiker-Biker  > Colmar Manor
• Bull Run Mountain from Reston Station.
• Patapsco Creek from the BWI MARC station
• Rosaryville from Largo Station
• 2 Day Bikepacker from DC
• Green Ridge State Forest via Amtrak. Probably a 2 nighter based on train schedules. Looks like a climbing challenge.
• Sligo Creek Trail > Matthew Henson Trail > Rock Creek Trail > Georgetown Branch Trail > Met Branch Trail (This is a fantastic trail loop that doesn’t get as much love as it should)
• Glenmont to the nearest visible Mason-Dixon marker 
• Loudon Gravel Lollipop out of Reston
• PPTC’s Backroads Century is pretty rail accessible from the Harper’s Ferry train station. Hmmm.
• PTAP MTB trails from BWI MARC Station – I got very lost last time I tried this. Plus the trails are not really suitable for underbiking.