By 2018, Metro riders will be able to take the Silver Line to Dulles International Airport.

The Loudoun County Board of Supervisors voted 5-4 Tuesday to extend the rail line through the airport and into the county after months of political wrangling and number crunching.

The county will have to pay at least $270 million toward the rail line's $2.7 billion second phase, which will extend the Silver Line from Reston into Loudoun.

The $2.9 billion first phase, which will pass through Tysons Corner and ends in Reston, is under construction and scheduled to carry passengers by early 2014.

State and local officials who support the project applauded Loudoun’s vote. If Loudoun had refused to join the project, construction and financing plans would have to be redrawn, delaying the rail line's completion by at least 18 months and adding millions of dollars to its price tag.

Virginia Gov. Bob McDonnell said through a spokeswoman he was "pleased" by the vote.

"This is a critically important project to both reduce congestion as well as spur economic growth and job creation in Northern Virginia,” said spokeswoman Taylor Thornley said.

Four Loudoun supervisors voted against the rail line after a testy meeting, saying they weren’t convinced Metro would provide enough benefit to justify the expense of building it and the roads, schools and other infrastructure needed to support it.

“There is no difference between the Vikings, who came to plunder and pillage… and Metro,” said Supervisor Eugene Delgaudio.

Supervisor Ken Reid, a one-time opponent of the project, cast the deciding vote in favor of it.

Reid said he still questions whether Metro would bring all the promised benefits, but was willing to support it because the board also agreed, in a 7-2 vote, to create a special tax district to pay for the rail line.

The tax district would include areas around the proposed Metro stations and would add up to 20 cents per $100 of the assessed value of occupants' property to pay for the rail line.