If you're looking for scenic cycling, try the path that follows the old Washington & Old Dominion Railroad roadbed through Arlington.

Barcroft Park, just south of Four Mile Run Drive between George Mason and Walter Reed drives, is a good starting point. Ride across the narrow bridge over the brook and you're on the bike path.

As you leave the park there are wetlands to explore as well as streams and woods where mallards nest and feed, killdeers raise their young and redtailed hawks hunt for mice and rabbits. Stop and listen for the songs of toads, frogs and woodthrushes.

Just south of the George Mason Drive underpass, there are deposits of carbonized trees (lignite) uncovered by the stream. Then it's past the Brittany, up the hill and down again and over the first ford crossing Four Mile Run under Columbia Pike.

As you pump your way up the hills to the next ford, you'll pass Huffman's Whitewater, which marks the fall line where coastal plain meets piedmont. This is also where the old Barcroft Mill stood; little remains but a marker.

You might want to stop in at Long Branch Nature Center. There's a marker at the junction of Four Mile Run and Long Branch commemorating the site of the oak tree upon which George Washington, as a surveyor for Lord Fairfax, carved his name more than two centuries ago.

Located in what was once Arlington's agricultural heartland, the nature area is today a quiet park in the middle of a growing urban community; the woodlands, meadows, streams and ponds are a refreshing relief from urban sprawl, and the center has displays and live exhibits. The self-guided Woodland Trail winds through upland meadow, stream and rock garden, and the Long Branch spur runs from Carlin Springs Road down into Glencarlyn Park, where it joins the bikepath.

Passing more picnic facilities and fording the run, you come to John Carlin's Springs, where a railroad passenger in the late 19th century was always happy to stop. A wooden marker stands at the spot where John Carlin built a small shelter and welcomed visitors with fresh lemonade, homemade ice cream and refreshing springwater. The springs are half a mile south of the Carlin Springs Road underpass in Glencarlyn Park.

A short detour north will take you past the Arlington County Amphitheater, known for its live summer dramatic performances. Back on the bikepath, you ride through Bluemont Park, where you can stop for a drink of water or soda or grab a game of tennis on the newly restored courts. You might also want to try the park's disc golf course -- it's like miniature golf with Frisbees.

Across Wilson Boulevard is Bon Air Park, a pleasant pause on the bikepath with its picnic facilities and outstanding rose garden. The azaleas bloom in late May, but the roses go all spring and summer.

Riding along I-66, you'll see a small waterfall just north of McKinley Street that has exposed this area's predominant bedrock -- mica schist. Below the falls on the south bank you'll find outcroppings of soapstone, a soft talc rock that was quarried by Indians to make cookware.

The path, maintained by the Northern Virginia Regional Park Authority, then leads through the summer backyard gardens of Falls Church after passing the site of the old Falls Church railroad station, now only a marker. You pass through downtown Falls Church, up the hill and over I-66 and the Beltway. Here are some good spots for viewing the building of the Interstate highway.

Along the bikepath following the old railroad, you go through old Vienna, across the 1904 railroad overpass and past the marker commemorating the first use of a railroad in warfare -- in an 1861 Civil War battle nearby.

Then you hit downtown Vienna, where visits to Freeman House and the model train museum in the old train station are well worthwhile. Freeman House, built in 1860, was first a general store serving the people of Vienna and nearby farmers. It also doubled as the town's first train station before the nearby depot was built. Restored in 1974, today it's a model of a Victorian store. Note, too, the nearby Presbyterian church, built in 1874, that helps make this part of Vienna look more like a New England village than a suburb.

Across the street (although the address is 231 Dominion Road NE) is the railway museum, operated by the Northern Virginia Model Railroaders. It's officially open only for shows, but you can visit whenever one of the members is working on the models -- just about anytime. There's a scale model of the railroad and a reproduction of a 19th- century ticket agent's office.

In the depot's auditorium are exact models of North Carolina's four main railroads, along with trolley, interurban and freight-train models and exact reproductions of stations, interchanges, bridges and tunnels. Even the mountains the trains travel over and through are to scale. GETTING THERE To get to Barcroft Park from the District by car, take Columbia Pike west and turn left on Four Mile Run Drive. Turn right into the lot just past George Mason Drive. By bike, take the Mount Vernon bikepath south from the District, just past the airport to Four Mile Run. Veer left and ride under the George Washington Parkway on the path along the stream, then up through Presidential Gardens, along Glebe Road, through Park Fairfax, over the Shirley Highway overpass and back down to the bike path at Shirlington and on to the park..