The Washington & Old Dominion Railroad Trail runs 45 miles, from Shirlington to Purcerville Virginia. As the name implies, it follows the old railroad grade for the Washington & Old Dominion Railroad, which ran between DC and parts of Northern Virginia from 1859 until 1968. It's a beautiful paved asphalt trail, 10 feet wide and for the most part, smooth, flat and fast.
When the W&OD Railroad closed in 1968, it owned a 100-foot wide right-of-way on either side of its 60+ miles of track. Soon after the railroad closed, the Virginia Highway Department purchased the railroad's property with the intent of using a portion of the right-of-way for the construction of I-66. After Dominion Power purchased much of the property that was left (to use as a corridor for the power transmission lines), the Northern Virginia Regional Park Authority (NVRPA) was able to lease most of the land for the bike section. The first portion of the W&OD opened in 1974, and was so popular that in the years since, the NVRPA had been able to purchase most of the 45 mile length of property.
The NVRPA is an unique organization that represents Arlington, Fairfax, and Loudoun counties, and the cities of Alexandria, Falls Church and Fairfax. While the trail is owned by the NVRPA, it's primarily maintained by the Friends of the W&OD, a local nonprofit organization that works as an advocate for the trail.
In addition to the towns I've already discussed, the trail passes through a huge number of historical places, many of which date to pre-Civil War times. Of course, there's also a lot of railroad memorabilia; bridges, abutments, mile markers, etc. There's even an old train car and caboose sitting around.
It's a fantastic way to recycle some of our past into a usable piece of society, and it's heavily used. As I ride west on the trail (away from town), there are very few cyclists headed my direction, but I pass 100+ cyclist commuting into town.
I've also noticed the overwhelming lack of trash. You don't see fast food wrappers, Starbuck cups, or other litter on the trail. It's an immaculate shape, and a great way to get to work.