Arlington Cemetery North can be reached through the Arlington/Wasteland Metro (Flooded Metro on the minimap, not by the Arlington Library) just south of Wilhelm's Wharf, a short bit southeast of Super-Duper Mart and close to the road to the Anchorage Memorial. It also connects to Georgetown to the East and Dupont Circle to the North. The connection is made through the Arlington Utility tunnel, which connects to the DCTA Tunnel 014-B Potomac tunnel that runs under the Potomac River and connects Georgetown West to Dupont West/Foggy Bottom.
Don't forget to visit Arlington House while you are there. The luck bobblehead is in the house, along with a 'Big Book of Science' stat book. There is also a safe in the master bedroom, partially hidden by the dresser.