Northern Virginia's longest biking, hiking and horseback riding trail -- along the former ywashington & Old Dominion railroad right of way -- will be dedicated Saturday with an all-day festival in Reston and Herndon.

The dedication marks the completion of several major improvements to the W&OD, including paving of the trail between Vienna and Herndon that creates 16 miles of bicycle expressway from Arlington to Loudoun County. Another 22 miles to Leesburg and Purcellville is rough gravel, now open to hikers and horseback riders.

The festivities, beginning at 10 a.m., will start along the trail near the A. Smith Bowman Distillery on Sunset Hills Road in Reston. A ribbon-cutting ceremony will be followed by a parade of bicycles, horses and pedestrians to Herndon. There, at 10:45 a.m., the trail officially will be dedicated by Rep. Frank R. Wolf (R-Va.) and Fairfax County Board Chairman John F. Herrity. Throughout the day, at Herndon or near the distillery in Reston, visitors will be treated to music, crafts and antique fairs and refreshment stands.

Bicyclists can now travel for more than 20 miles from the Potomac River to Loudoun County, although the existing trail in Arlington, following Four Mile Run, is in poor condition. Alexandria and the Army Corps of Engineers recently paved almost two miles of bike trail along Four Mile Run in the city, including a bikeway under Rte. 1 and a half dozen railway bridges.

With the construction of 11 new bridges and extensive trail work this winter, the Northern Virginia Regional Park Authority has now spent about $5 million -- including $1.6 million in federal funds -- on what it concedes is the skinniest park in Northern Virginia, a narrow bike trail that has replaced the once existing tracks of the W&OD railroad.

The 100-foot wide regional park ultimately will stretch 42 miles from the Potomac River to Purcellville, in the foothills of the Blue Ridge Mountains. But no other major improvements to the trail are expected for several years because the park authority has run out of money.

"We'd like to pve to Sterling Park, an additional three miles, but that would cost about $75,000 and right now we just don't have it," Park Authority official David Hobson said this week.

The newly paved portions of the trail already have become popular biking and jogging routes between the dozens of Northern Virginia neighborhoods it connects. It is jammed on weekends and crowded almost every morning with joggers, bicyclists, school children and, occasionally, mothers with baby carriages.

Like the Mount Vernon trail, which is parallel to the Potomac River, the W&OD has hazards. The major ones are road crossings and the intermingling crowds of bicyclists, joggers and walkers. The lack of many traffic, street or trail signs can make it dangerous at road crossings and occasionally make it difficult for strangers to find their bearings.

The three most dangerous crossings appear to be at Rte. 7 in Falls Church, Rte. 123 in Vienna and Eldon Street in Herndon, four-lane roads where traffic is heavy.

The state highway department has approved a pedestrian-activated traffic light on Herndon's Eldon Street, although the town has yet to approve its installation. On Rte. 7 in Falls Church and Rte. 123 in Vienna, trail users are directed to detour to nearby traffic lights but many bicyclists and joggers have been darting directly across traffic.

Crossings at several other state highways, particularly the Rte. 15 bypass around Leesburg, also pose future problems. Trail users must now cross the bypass on foot. The state already has built a bike trail over the Beltway and crossings for the W&OD on routes 7 and 9 beyond Leesburg.

The W&OD has one minor hazard that is never seen on the Mount Vernon trail: horses. West of Vienna the W&OD has a parallel horse trail, although horses occasionally share the trail with bikers and pedestrians.

Another summer hazard is heat. There are virtually no trees or shade along the trail, since much of the trail is under the power lines of the Virginia Electric and Power Co., and Vepco has the right to cut any trees that grow more than 15 feet high within the right of way.

The park authority plans limited landscaping along the trail. But because of insufficient funds it expects to let nature takes its course in most places, at least to within 15 feet of the trail's edge said Hobson.

Already, nature is doing pretty well. A rider last week found many parts of the trail thick with blooming wildflowers, dogwood and azaleas.