What's 100 feet wide, 42 miles long and stretches from the Potomac River to the Blue Ridge Mountains? It's the Old Dominion Railroad Regional Park, often called the skinniest park in Virginia.

Thanks to a detailed new trail guide that can fit in a back pocket, it's now easier to explore the scenic route dotted with historic buildings and near other regional parks. Most of the trail is paved and suitable for bicycling, jogging and hiking.

Built on the roadbed of the former Washington and Old Dominion Railroad, the park begins near the Potomac River in Arlington and travels through Reston, Herndon, Sterling and Loudoun County, ending at Purcellville. The W&OD railroad ran along the same path from 1859 to 1968, and during the Civil War it was almost destroyed.

Train passengers who traveled the route from Union Station in the District of Columbia called it the "Virginia creeper" line. Park authorities say that name is still used affectionately to describe the narrow, winding trail.

The new trail guide offers 19 detailed maps, covering each portion of the trail and surroundings. It gives a history of places such as Falls Church and Vienna, including the fact that the first tactical use of a train in military conflict took place along the Vienna stretch of the W&OD rail line on June 17, 1861. A plaque erected beside the trail commemorates the event.

The guide points out flowers, trees, birds and other wildlife that travelers may see along the trail. And for horse enthusiasts, the park authority has constructed a separate, bluestone-surface bridle path that parallels the trail for 23 miles from Vienna to Leesburg.

Bicycle repair shops, restrooms, fast food restaurants and other services along the trail are listed in the guide.

It is available for $2.50 at all regional parks in Northern Virginia, or for $3 by writing the Northern Virginia Regional Park Authority, 5400 Ox Rd., Fairfax Station, Va. 22039. It is also available at bicycle shops in Northern Virginia and 7-Eleven stores along the trail. Call 352-5900 for more information.