At one point in time the DC area had a streetcar system with over 200 mile of tracks (Metro rail has just over a 100). Parts of the system, completely abandoned, have been used to create the College Park Trolley Trail and the North Bethesda Trail. Small pieces exist here and there throughout the region, including almost one entire line from Georgetown to Cabin John.
The Washington and Great Falls railway, aka the Cabin John Trolley or the Glen Echo Trolley, was opened in 1895 and shut down in 1960, but its narrow width (making it unsuitable for roads or metro rail) and precarious location - shoehorned in between parks and aqueducts on the side of a ridge - have left it largely untouched. The right of way basically exists in two pieces, one in Maryland and one in DC.
The section of the right of way in Montgomery County, from Brookmont to Cabin John, has been proposed for a bike trail on numerous occasions, most recently as part of the MacArthur Boulevard Bicycle Improvement Project, but that was dropped from the plan in January 2005.
Part of the right of way in DC, the 0.6 miles from Foxhall Road to the Georgetown Reservoir, was included in the Canal Road Scenic Byway Plan. North of that, the right of way crosses Reservoir Road, through Battery Kemble Park into the Palisades Neighborhood almost all the way to the Capital Crescent Trail.
South of Foxhall Road, the right of way passes over Glover Archbold Park (and Foundry Branch) on an abandoned trestle (pictured) and from there passes just in front of Georgetown University. While the 0.2 mile piece may seem small, the Southern Section should be made into a bike/ped trail.
The land is owned by WMATA, passes through NPS property and onto Georgetown's campus so there are a few serious stakeholders here. The small section stands on its own, as it creates a short path to Georgetown from the Palisades area - without having to go down the embankment and back up. The sidewalk along Canal Road is narrow and unpleasant. But as part of a complete trail it's even better.
The complete trail (which you can hike or bike right now if you don't mind carrying your bike at times) would create a 2.5 mile rail trail along the bluff overlooking the Canal and the Potomac. It would parallel the CCT trail providing an alternative and compliment to one of the busiest trails in the country. Unlike the CCT, it connects into the city grid - being on the city side of the Canal and Canal Road.
Unlike everything else on this list, I'm actually working on this. DDOT seems, at least, intrigued and may have money for it. Rails to Trails has come out to look at it and is also supportive. The 0.2 mile section is going to be the easiest section (although possibly the most expensive) to build - namely because it has no neighbors. Sometime in the spring, you can expect to see a "ride" advertised on this blog and also a coalition building meeting.
So there's something to look forward to.