A Sunday article by the Washington Post sheds some light on the reasons for the delay.
Per the article, it was found during system tests conducted on Thanksgiving weekend that changes needed to be made to the Automatic Train Control system software.
"During those tests, officials concluded that changes had to be made to the software that governs what is known as the automatic train control system ... The software changes required will ensure that the computers that help run the system communicate clearly with each other.
The ATC system controls train movement and speed and ensures proper spacing between trains traveling along the system’s more than 200 miles of track. If a train is traveling too fast or gets too close to another train, the system will slow or stop the train if the operator does not act.
The ATC system is critical to the safe operation of Metro’s trains. It was the failure of this system to detect the presence of a train on the tracks that was blamed in part for the 2009 Red Line crash that killed nine people and injured dozens of others."
Once the fixes are made, the Metropolitan Washington Airports Authority will conduct tests that simulate a regular day of service, to include 80 trains on the line, according to the article.
The Silver Line Phase One will terminate at the Wiehle Avenue Station in Reston until at least 2018, when the second phase is expected to go further west along the Dulles Toll Road with additional stops in Reston, Herndon, Sterling, Dulles Airport and Ashburn.