JM from Central Florida on 8/6/2020 8:04:33 PM:
I biked Pittsburgh to DC 2 weeks ago. I parked at Great Falls Tavern, MD (then biked to DC Reagan National Airport, rented a car, drove to Pittsburgh, dropped the car off in downtown Pittsburgh, rode to Point State Park and then back to my car at Great Falls). Parking at Great Falls Tavern is plentiful and free. If you alert the NPS either online or at the entry gate, you'll feel better about leaving your car.
I considered Amtrak, but recent info made me wonder how that would work out. It sounds as if the bike racks aren't currently being used, but bikes are laid down in the baggage cars. There have also been recent notices on the Amtrak site about possible bus transfers due to construction if the train got behind schedule by more than an hour. Too many unknowns for me. Good luck and enjoy the trails.