The 42-mile biking-hiking trail from Alexandria to the Blue Ridge Mountains, acquired this week by the Northern Virginia Regional Park Authority, follows what's most popular vacation routes - along the rickety rails of the Washington and Old Dominion Railroad.

Although the tracks were laid before the Civil War, the railroad was a success only for a brief period during the World War 1 when as many as 10 trains a day trundled over W & OD tracks to mountain sorts and a spur route to nearby Great Falls billed by railroad posters as "the Niagara of the South." More than 10 million passengers a year jammed the trains in 1918 to see the natural wonders of the Virginia countryside.

The automobile mania of the 1920s emptied the trains, however, and the railroad resumed its downhill course, finally going out of business in 1968 after it had been reduced to a single freight train a day, which traveled at a maximum speed of 15 miles an hour over wobbly, weedy tracks. In honor of its speed, and an ivy-like vine, the W & OD railroad was called te Virginia Creeper - the name chosen for newest and longest of Northern Virginia parks.

The 42-mile, 100-feet wide W & OD right of way, bought in 1968 bu the Virginia Electric Power Co. for its power lines, has remained an inviting ribbon of green and open space through Washington's fast-developing suburbs. Now, under park authority plans, in two to five years it will be filled with trees, shurbs and flowers, and once again will be turned into one of Washington's most popular "transportation corridors," this time for bicycles, horses, joggers and hikers.

The bicycle trail already is visible in Falls Church, where Vepco allowed the city to build a bike path under its power lines two years ago during negotiations for sale of the W & OD right of way. A neighborhood eyesore filled with weeds and trash when the city and local garden clubs went to work on it, the right of way now is a landscaped grassy area with azaleas and other shrubs and trees banked along the eight-feet wide, paved bike path.

"It's terrific. Our kids learned to bike on it and they use it all the time . . .in fact it's the only safe place for them to bike," said Sheila Johnson who lives 20 bike lengths from the trail at 801 Lincoln St. "We've ridden all the way into Arlington on the trail and think it's a great addition to the neighborhood. The only problem is that it's too dangerous to cross Broad Street (Rte. 7), so we don't go down that way."

Road crossings will be among the new bike route's major hurdles, since it must cross the Capital Beltway, I-66, numerous four-and five-lane roads, such as Rte. 7 and Rte. 123, and dozens of smaller roads and streets between Alexandria and Purcellville in Loudoun County, where it will end.

"The Virginia State Highway Department is being unbelievably cooperative on this," says park authority executive director Darrell Winslow. "They're going to build bridges over both I-66 and the Beltway for us by the end of next year." Button-activated traffic lights are among the devices envisioned to help trail traffic cross major roads and highways.

The eastern part of the trail will end in the Shirlington section of Arlington at I-395. But Arlington planners say the U.S. Army Corps of Engineers is building a bike underpass beneath the 10-lane interstate, along the Four Mile Run Flood plain, and beneath Rte. 1 and the Potomac Railroad Yard.

Money also is a major problem. Although the park authority is buying the right of way from Vepco for $3.5 million - considered a bargain price since Vepco apaid $5 million for it and outside appraisers recently valued it at close to $10 million - it will take an undetermined amount of additional funds to build the adjacent bike and horse trails and to landscape the entire route.

The park authority has applied for a major grant under the federal Bureau of Outdoor Recreation's new $5 million "Rails to Trails" program, and park officials hope area businesses and garden clubs also will contribute to the regional park where it passes through their neighborhoods.

The extent of outside support will determine how fast the new bike trail is completed. The entire route could be finished within two years if a large federal grant is received but could take as long as five years if no federal help is obtained.

The first section to be completed will run from Falls Church to Vienna, over I-495 and I-66, and the park authority expects the section to open within 18 months.

If an appropriate house becomes available along the right of way, American Youth Hostels has expressed an interest in operating an overnight hostel for bikers taking long trips on the trail. For hikers, the trail will end an few miles from the Applacian Trail for hikers, and bicyclists will be able to pedal up to nearby Harper's Ferry and the C &O Canal tow path to make a 120-mile circle tour of the Virginia and Maryland countryside.

Several of the W & OD railroad stations are still standing - Herndon city planning officials have offices in their old stations and Vienna's is now being restored - and the park authority hopes to use one or more as "way stations" where bicycles may be rented and snacks served to trail users.