THE GREAT DIVIDE has been crossed. The way is open to the West. The W&OD Railroad Trail has reached Leesburg.

With the paving of a bicycle trail and construction of a parallel horse trail now finished into downtown Leesburg, 35 miles from Alexandria, the Northern Virginia Regional Park Authority has completed the crucial section of the longest paved bike trail in the East. And the gravel horse path beside it has opened the way for long-distance trail riding.

Horse people already are trotting into Leesburg for Sunday morning breakfasts, tying horses to fences and lamp posts and again giving the town a horsey, 19th-century air.

And last weekend hundreds of cyclists pedaled into Loudoun's county seat even though the paving wasn't quite finished. Children on Christmas dirt bikes. Elderly riders on old three-speed Raleighs. Club riders in shiny black shorts, striped shirts, helmets and heel- less bike cleats. The club riders waddled clumsily on the ground outside Leesburg's Red Apple Deli -- 80 percent of the new deli's weekend business is cyclists, says cashier Denise Rogers -- but then with cleats flying at 110 rpm, swept swiftly past on the trail, silent except for the singing of their imported silk sew-up tires.

Leesburg is not the last stop on this 100- foot wide railroad trail, the skinniest park in Virginia. The park authority hopes to pave 10 miles farther to Purcellville within the next three years and then some day extend the W&OD an additional five miles to the Appalachian Trail and the Blue Ridge town of Bluemont. Now only hikers and horses can follow the trail beyond Leesburg.

But to reach Leesburg is a major recreational breakthrough.

The W&OD now provides a wide, paved trail cutting an almost straight swath across Northern Virgini's sprawling suburbs and its traffic-jammed roads.

It is the only direct and safe way for cyclists to get to the scenic heart of Loudoun County without traveling on high-speed highways like Route 7 and U.S. 50 -- dangerous even for experienced cyclists -- or riding on gravel and dirt back roads. Loudoun has more of those than any other county in Virginia.

Historic Leesburg is now within cycling distance -- a round trip of 30 miles from Herndon and 75 miles from Washington (for the more hardy or foolhardy). And from Leesburg you can ride out on lightly traveled back roads to peaceful old towns like Waterford, Hamilton and Hillsboro. You can keep heading west to the Blue Ridge Parkway and the Shenandoah Valley. Or you can easily cross Whites Ferry into Maryland, pedal down the C&O Canal Towpath toward Washington or meander along the lovely rural byways of western Montgomery County.

But crossing the flat Piedmont Plateau to Leesburg in the summer can be hot going: hot on bicycle, hotter on horses and hottest and slowest on foot.

The stretch between Herndon and Leesburg is largely treeless. Stretches of the trail may vanish in the midday haze, and distant cyclists and horses appear and disappear like mirages in the shimmering heat. The park authority should post signs such as: "Last Water for 10 Miles" or "Last Shade Tree Till Ashburn." Water bottles are almost a necessity for the crossing.

But while the Virginia Electric Power Co. has cut down all trees beneath its power lines -- Vepco's high-tension lines extend almost the entire length of the W&OD -- there are still a profusion of wild flowers and birds to admire from the saddle of your bike or horse. Daisies, chicory, morning glories, Queen Anne's lace, soapwort and snapdragons abound beside the trail. In the air are flurries of birds: goldfinch, thrushes, fly catchers, even indigo buntings. Riders must be alert for low- flying grouse or scurrying woodchucks crossing the trail.

(There is such a profusion of wildflowers and birds that the park authority will conduct a wildflower walk this Sunday, starting at 9 a.m. in Ashburn, half way between Reston and Leesburg. A guided bird walk, with a maximum 16 birders permitted so as not to outnumber and outshout the birds, will be held Saturday evening, August 10, from 7 to 9 p.m. in Reston. For information, call the park authority at 528-5406.)

And wildflowers are not all you get at Ashburn. The Partlow Brothers General Store, which has been providing food and refreshment at this country crossroads for 38 years, catering in the old days to farmers and W&OD train riders, now stocks its shelves for riders on bikes and horses, says owner Calvin Partlow. Juices, fruit and huge supplies of sodas await trail riders. An outside spigot and hose are for waterbottles and a quick shower. Recently Partlow received two portable toilets from the park authority to help the store cope with the growing crowds.

Brightwell Danewood, 73, an Ashburn resident who rides his horse regularly into Leesburg for Sunday breakfast, praises the park authority for its trail blazing but complains about the dangers of the narrow bridges where bicycles and horses cross together.

"Cyclists sometimes come so close I could kick them or the horse could," said Danewood last weekend. "You know this thing (his 21- year-old horse, Prince) isn't mechanical . . . and if a horse is startled, it could be dangerous on those bridges and embankments."

The railroad trestles, which the park authority rebuilt as part of its $7 million trail acquisition and development, have strong safety fences but no barriers yet on the high embankments leading to the bridges. Goose Creek bridge, west of Ashburn, is about 75 feet above the stream that provides drinking water for residents of Fairfax City and Herndon.

Horses drink it untreated and cyclists already are cooling their heels in its shallows. The stream is also a popular swimming hole. For drinking water, however, the park authority suggests that trail riders fill up at the small but growing number of water fountains along the trail -- many now being provided as a public service by the George F. Warner Plumbing Co. -- or at commercial watering holes like Partlow's and fast-food stores near the trail.

Two new bicycle shops have opened directly beside the trail -- Leesburg Bicycles and Sterling Bicycles. Both keep weekend hours and provide full-service repairs. Neither makes trail calls, however. But most cyclists, especially club riders who have tools and expertise, will stop quickly to offer assistance if you fall or break down.

The park authority has not finished work on the new section of the trail. Signs are not up (such as Yield to Horses signs). No landscaping has been done west of Herndon. There is work still to do in Leesburg.

But as Alexandria cyclist and computer consultant Harry Newman said taking his eight- year-old daughter on a tandem bike ride from Herndon to Leesburg for lunch the other weekend: "It's just fantastic that it exists at all. Oh, some parts are isolated and if you have a breakdown you've got a long walk. And my daughter says her legs are tired from the ride . . . But the park authority has done a good job."

ON THE TRAIL -- For further information about the W&OD, call the Northern Virginia Regional Park Authority at 352-5900. New detailed maps of the W&OD are now available for about $3 each at parks, many bike and convenience stores along the trail and by mail from the park authority at 5400 Ox Road, Fairfax, Va. 22030.