The 5-4 vote to opt-in to Phase 2 is the last needed from the project's four partners: the Metropolitan Washington Airports Authority, the Commonwealth of Virginia, Fairfax County and Loudoun County.

In reaction to the vote, WMATA Board Chair Catherine Hudgins released a statement congratulating Loudoun County for its "historic vote."

"We look forward to continuing to work with Loudoun County as an important partner at the table. The Dulles extension of the Metrorail system will support growing transportation demands, putting Loudoun County and Washington Dulles International Airport on the Metro map. Today's vote enables the long planned extension of the Metrorail system along the Dulles Corridor, with the companion benefits, to now proceed," said Hudgins.