WASHINGTON (CBSDC/AP) — Two northern Virginia congressmen want to study the possibility of extending the Metro transit system to Centreville in western Fairfax County and Woodbridge in Prince William County.

Democratic Reps. Gerry Connolly and Jim Moran say population growth in both counties and the expansion of Fort Belvoir will make additional transit options essential for the region.

There’s no timetable for completing the study, and it’s not clear how much an expansion would cost. The project would use existing rights-of-way.

The original Metro plan from the 1960s called for extending the rail network as far south as Prince William.

Metro is in the midst of a major expansion to Dulles International Airport.

Phase One of the project runs through Tysons Corner to Reston and is expected to be completed later this year. Phase Two still needs final approvals but would include three stations in Fairfax County and three in Loudoun County, including one at Dulles Airport.

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