Officials and some civic activists in Fauquier County reacted cautiously to Gov. James S. Gilmore III's plan to bring commuter rail service to their largely rural area, saying they're not sure they want to pay taxes to support it and worrying that it might speed development.

Under the proposal unveiled yesterday, the state would foot the bill for approximately $100 million in new track, switches, signals and trains to extend a Virginia Railway Express line from Manassas to Bealeton, in southern Fauquier. Gilmore (R) also wants rush-hour express service on that line and on the line that runs between Washington and Fredericksburg.

But Fauquier would have to help pay operating costs for the new service, joining a consortium of Northern Virginia counties that support the commuter line, which operates trains from Washington's Union Station to Manassas and Fredericksburg. Railway officials said yesterday that Fauquier's share of the expenses could reach $200,000 a year, revenue traditionally raised by levying a 2 percent gasoline tax.

"I don't think right now there is any will at all to raise the tax rate," said Larry L. Weeks (R-Scott), chairman of the county's Board of Supervisors.

Slow-growth activists also challenged the proposal. Southern Fauquier County still has vast tracts of farmland, and environmental groups are trying to protect it from encroaching growth from Prince William County. The debate over extending the railway will probably center on how much development it would create.

"This is one of our farming centers, and it's not in the [county's master] plan to have that high-population density," said Josephine F. "Jolly" deGive, director of planning services for the Piedmont Environmental Council.

The new rail service would represent a long-awaited boost for VRE, which is emerging from a two-year slump during which track work and a freight train derailment cost it hundreds of riders. Ridership--now 4,000 commuters a day--surged by 15 percent in the first quarter of this year, compared with the same period in 1998.

Railway officials say nearly 150 of those riders commute from Fauquier County, driving into Prince William each morning to board the train at the Broad Run or Manassas station. VRE spokesman Matt Benka predicted that the number of Fauquier riders would quickly double with the new service.

To coax commuters in the far edges of the region from their cars, VRE would also add two express trains on each line at rush hour, lopping as much as 20 minutes off what is now an 88-minute trip to Washington from Fredericksburg and 68 minutes from Manassas. A one-way ticket from Bealeton to Washington would cost about $6.70, officials said.

Benka said VRE has been limited in how much it can expand service because it shares tracks with freight lines. But he said the freight lines have agreed to allow VRE to expand service if the commuter line builds new track, as called for by Gilmore.

"Our trains are starting to come to a point where there isn't a whole lot more room to put people on them," Benka said.