ALL RIGHT, weekend readers, try this one: What's 100 feet wide, 42 miles long and mostly green? For the answer -- and a firsthand look at it -- we direct you this morning to an official dedication and celebration of "the skinniest park in Virginia," an offbeat and imaginative conversion of an old railroad track into a practical, attractive recreational strip through the "urban heartland" of Northern Virginia.

Longtimers remember this narrow strip as the route of the original "Virginia Creeper" -- the Washington & Old Dominion Railroad trains that ran along the right of way from 1853 until 1968. Today, thanks to some innovative pooling of ideas and money by the Northern Virginia Regional Park Authority, there is a park that is attracting national attention as an example of creative land use. With appropriate hoopla and ribbon-cutting set for 10 a.m. in Reston and 45 minutes later in Herndon the last main section of the trail will open for bicyclists, joggers, horseback riders, hikers and strollers and anyone else who isn't "motorized."

When all is said and done, the trail will stretch from the Potomac River to the Blue Ridge, connecting with other trail systems in Falls Church, Arlington and Alexandria, as well as with the National Park Service's bike trail along the river in Alexandria, the C&O Canal bike path and the Appalachian Trail. Along the way will be neighborhood playgrounds, soccer fields, refreshment stands, picnic groves, hostels and, we assume, more than a few enthusiastic users.

Some of this is still to come, but today the park will abound with artists, craftsmen, musicians, baton twirlers, baseball players, horseshoe tossers and those who are content simple to soak in the surroundings in more quiet zones. There will even be lemonade and poppcorn. Whatever one's pleasure, this resourceful rendering offers the region a singularly attractive park in place of a tired strip for old train tracks and heavy-duty electric power lines -- and therein lies a jolly green bargain.