The Urban Mass Transportation Administration agreed yesterday to provide federal funds for commuter rail service between Fredericksburg, Va., and Washington, apparently clearing the way for a one-year trial of the plan to begin within a few months, according to Rep. Stan Parris (R-Va.).

The trains will serve commuters in Lorton, Springfield, Alexandria and Crystal City, with a Washington stop at Union Station. Northern Virginia commuter rail service could begin as early as Jan. 12, according to Parris.

Under the proposal, daily service would consist of one Fredericksburg-to-Washington morning train and one evening train going in the opposite direction. But if the one-year experiment is successful, commuter rail supporters hope to expand the rail service to include four trains daily between Manassas and Washington.

Parris said federal transportation officials have agreed to provide 75 percent -- or between $750,000 and $1 million -- of the capital cost of the one-year commuter rail experiment. State or local funds are needed to pay for the remaining costs. The concept has been endorsed by both state and local officials.

The commuter rail service will be operated by Amtrak on existing tracks, so that no new tracks are required. In addition, no new trains will be purchased, because officials plan to use a train from Amtrak's "The Virginian" run, scheduled to be discontinued this fall.

Local supporters of the service have argued that a commuter rail system could help alleviate the pressure on Northern Virginia's major roads, which are clogged with commuters during rush hours. Critics, however, have questioned how extensively the limited system will be used.

The UMTA construction grant was required because Amtrak said it needed funds to build platforms and parking lots at each of the commuter stations.

The operating costs for the first year are estimated at $1.2 million. Officials expect fares from commuters to generate about $400,000, and the balance is to come from the jurisdictions served by the trains.

Fairfax, Prince William and Stafford counties have endorsed the commuter rail concept, although Fairfax transit officials have cautioned that the rail could turn into a risky and costly endeavor.

Parris has called a Friday meeting with local officials from all the jurisdictions involved to brief them on the plan.