There are no restrooms and few water fountains alongside the W&OD Trail in the County. Restrooms and additional water fountains are available near ball fields and picnic areas along the Four Mile Run Trail. The W&OD Trail's trailhead (Mile 0) is at the intersection of South Shirlington Road and South Four Mile Run Drive. The trail starts in the Atlantic Coastal Plain at the trail's lowest elevation: above sea level. The trail climbs in while traveling northwest through Arlington County. While in Arlington, the trail ascends through the Atlantic Seaboard fall line while climbing upstream in the valley of Four Mile Run. The trail crosses the Run seven times in the valley on bridges whose abutments were constructed before the Civil War by the Alexandria, Loudoun & Hampshire Railroad, a predecessor of the W&OD Railroad. After crossing Columbia Pike (VA 244), the trail enters a steeply-sloped woodland that covers both sides of the valley. The trail crosses under Arlington Boulevard (U.S. Route 50) while within the woodland. Near the end of the woodland, the trail intersects the Bluemont Junction Trail, a long paved rail trail that travels to Ballston on the bed of a former W&OD Railroad branch that once ran to Rosslyn and Georgetown. past that intersection, the W&OD Trail intersects the Custis Trail, a long paved shared use path that travels along Interstate 66 (I-66) (the Custis Memorial Parkway) to Rosslyn and which provides access to Washington, D.C. and the northern end of the Mount Vernon Trail. After crossing its intersection with the Custis Trail, the W&OD Trail travels northwest near an I-66 soundwall for most of its remaining course in Arlington. After crossing under North Sycamore Street next to Four Mile Run, the W&OD Trail crosses over the Run and passes the East Falls Church station on Washington Metro's Orange and Silver Lines. After leaving the fall line, the W&OD Trail enters the Piedmont plateau region near the west corner of the County. The trail continues to climb in the Piedmont while traveling northwest through the City of Falls Church and Fairfax County. The trail reaches an elevation of at its crossing of I-66 in Fairfax County. When it reaches this point, the trail has risen while traveling . After crossing over I-495 (the Capital Beltway) on a trail bridge and passing Dunn Loring, the trail begins a long descent as it travels through Vienna. After leaving