Sealston/Fairview to Dahlgren Lab - US Navy
Former US Government (US Navy) railroad right of way facing east from Va. Route 605, less than a mile east of the "end of branch". 
Photo from Mike Palmer, May 2003.
CSX operates a former Richmond Fredericksburg & Potomac branch east from Dahlgren Jct. (just north of Fredericksburg) to just east of Sealston. There are a couple of freight customers near Sealston; this line is north of and parallel to Va. Route 3.

East of Sealston the branch was operated by the US Government (US Navy) to support the Dahlgren Laboratory on the western bank of the Potomac River.  The US Navy segment has been abandoned for some time, and land in the area is posted "No Trespassing". 
The forest has completely taken over the right of way at this point!  This is at the same location as the photo above, however it is facing West.
Look closely in the right-hand third of the photo...there is a standard metal crossbuck with a wooden post, with tree branches surrounding it.
Any trace of rails, ballast, etc. is completely gone (or buried) at this location.
This is at Va. Route 605, just north of the junction with Va. Route 665 in King George County.

Photo from May 2003 courtesy Mike Palmer.