Nine Loudoun County supervisors on Tuesday will decide the fate of Metro's $6 billion Silver Line.

The county board will vote on whether to pay at least $270 million to extend the Metrorail line into the county.

If Loudoun officials decide against the rail line, it could mean a delay of 18 months and millions in extra costs -- and halt in Reston what was always envisioned as a rail line to Washington Dulles International Airport.

The four supervisors who favor the rail project need only one more vote from the all-Republican board to extend the Silver Line to the outer suburb. And observers say Supervisor Ken Reid, R-Leesburg, might supply that decisive vote.

But Reid said Monday that he still wasn't sure how he'd vote.

"I have to see how the motions are [Tuesday]," he told The Washington Examiner. "I told them I'd think about it."

The rail line has been the source of bitter contention in the county for months as residents and officials debated costs and benefits.

Rail opponent David LaRock said the Silver Line will bring traffic, crime and higher taxes to Loudoun.

"If it was a reason-centered discussion, this project would have died a long time ago," he said. "[Metro crime] is going to mean a lot to the people of Loudoun if they really begin to understand that. It's not just this Disney-like train that floats around and makes everybody happy."

Business groups and Virginia Republican Gov. Bob McDonnell have been pushing Loudoun to approve the project, saying it would lead to billions of dollars in economic development over coming decades.

Even if the board says yes to the rail line, the fight could continue. LaRock, a member of Loudoun Opt Out, said his group may seek other ways to stop the rail line, including a lawsuit.

Some of those opposed to the rail line said that even if they reject it now, Metro may still someday come to Loudoun.

"If we didn't opt in, I don't believe this would be Loudoun's last opportunity to sign up," said Supervisor Geary Higgins, R-Catoctin. "Why would it be the end of the line? They've extended lines all over the [Metro] system."