Got this message on my neighborhood list serve.
"From Arlington County: The S. Park Drive entrance to the W&OD trail will be closed for two days to conduct survey work related to Sparrow Pond. The closure will take place sometime between August 25 and September 1."

This trailhead is the one you might use perhaps to get to Ballston (via Quincy/Henderson), especially for Conte's Tuesday night ride that starts out of Mosaic Park.