Connecting from just south of the Long Bridge and running east of Jefferson Davis Highway (Route 110) to Arlington Boulevard and then west of the GW Parkway into a terminal between 19th Street and Lee Highway was a spur of the Pennsylvania Railroad. This freight line originally served as a connection between Potomac Yard and the W&OD's Bluemont Division. I'm not sure when trains stopped running on this section or when the tracks were removed. The W&OD stopped in 1968 and shortly thereafter the Bluemont Division sold to Virginia to build I-66. The Theodore Roosevelt Bridge, which built on the line's norhtern footprint was built in 1964, so it's not unreasonable to think that that part at least stopped running then. But, if you look on the local area map at Pentagon Metro you can still see a black line for the southern part of the rail line on the map
This old rail line doesn't actually leave much of an opportunity.
The TR bridge spills out over the Virginia side between Route 110 and GW like the business-end of a pom-pom, making it extremely expensive to traverse. A trail could be built from the west side of the GW bike-ped bridge along the west side of the Parkway and then back west along the Arlington Boulevard ramp up to North Lynn Street, but I'm not sure what purpose it would serve.
Between the TR Bridge and Washington Boulevard the right-of-way (ROW) was mostly taken over by Metro. Between the Metro Station and Washington Boulevard, a social trail already exists along the ROW and a formal trail has been in the plans for 14 years [last I heard, in June 2007, the sruvey was complete and the design was underway]. If the TR bridge's south bike/ped lane were connected to something on the Virginia side - which the bike plan calls for, one would assume that would mean a grade separated crossing of the Arlington Boulevard to GW Parkway lanes. This would allow for a bike trail to run from that point, between the Metro line and the GW Parkway, to Memorial Drive (and the Route 110 trail).
South of Washington Boulevard the ROW used to run between 110 and the Pentagon Parking lot (if you look at the aerial photos on Google Maps as of the writing of this, you can still see what it used to look like). But when Route 110 was moved farther away from the Pentagon following 9/11, the ROW was bisected twice (you can see the after shot here at MSN maps). Furthermore, Route 110 and Boundary Channel were brought side by side. Pedestrian bridges were built over Route 110 (Hooray) but I don't think they're open to the public (Drat). It would be possible to cross Boundary Channel Drive and build a path on the east side to the Pentagon overlook of the Washington Channel where one could connect to an existing sidewalk that, together with a marked crosswalk, connects one to the ROW. This would actually have some value if the Humpback Bridge underapass were connected to Boundary Channel Drive - and the same aforementioned sidewalk. Not that Boundary Drive isn't bikeable, you could just dump bikes onto that road but, of course, you'd have fewer takers.
This leads to the final piece of the ROW and the only extant element. The tunnel under 395. This tunnel still exists and it appears to be used for storage of heavy equipment. On the NW side is the Pentagon Power facility (restricted access) and on the SE side is a personal storage facility. Access is from the Pentagon side only as a fence is on the other side.
When the Long Bridge Park is finally built, and with it a connection to the Mt. Vernon Trail, it would be nice to have a piece of this tunnel opened up for cyclists. The problem is, no surprise, the Pentagon Power facility. It may be physically possible to squeeze a trail along the south side of Boundary Channel Drive and the ramp from there to I-395 south. This would involve reducing the width of the shoulder and cutting down the trees that hide the Pentagon facility, then after making it around the corner of the NE building, drop the trail down and into the tunnel where a fence or wall would separate trail users (who would get 10-15 feet) from the equipment storage. But, there nearly no chance of any of that happening in my opinion. Once through the tunnel, connecting to the Long Bridge Park trails would be simple.
1) The Route 110 trail is in the plans and makes sense. It should be built.
2) A connection from the north end of that to a potential TR Bridge connector would be great. If the connector is ever built, so should the extension
3) Continuing the trail south to the Humpback bridge underpass, again if it were connected to Boundary Channel Drive, would be a sensible addition.
4) Building another spur off the south end through the I-395 tunnel to Long Bridge Park would be useful but physically complicated and politically improbable.
5) A connection from the GW bike/ped bridge to North Lynn Street, while physically possible is redundant and perhaps not so useful.