Also Washington Street in Alexandria is part of the Parkway and both Alexandria PD and the U.S. Park Police have joint jurisdictiion along it. Under current regulations this MAY constitute a SECOND "Felony Furlong". Perhaps some of our Legal Beagles could sniff this out?
In any case if you are broen down or stopped by an LEO in a "felony furlong", be aware of your situation. As I understand the situation, BTW, it is impossible to carry a weapon openly or otherwise into Old Town below Washington Street without crossing the GW and being (momentarily) in violation of Federal law.