The Metropolitan Washington Airports Authority says more tests are needed on Phase 1 of the Silver Line.

Necessary software modifications for the automatic train control system are the reason for the latest delay. Those updates will be followed by more testing, delaying the transfer of the Silver Line’s operations to Metro.

Last month, the Airports Authority said it expected Phase 1 to be substantially completed and ready to turn over to Metro by the end of November.

“We will not do anything that might compromise safety or create unsafe conditions for the traveling public,” said Pat Nowakowski, executive director of the Dulles Corridor Metrorail Project.

In July, the target date for substantial completion was pushed back to November after additional work and testing needed to be completed on several items.

There has been no date announced for when actual passenger service will begin on Phase 1 of the Sliver Line.

Phase 1 runs from the current East Falls Church Metrorail stop on the Orange line, with four stops in Tysons Corner. Early work on Phase 2 of the Silver Line, from Wiehle Avenue into Loudoun County, with a stop at Dulles International Airport, is already underway.