Northern Virginia officials learned details yesterday of new commuter rail service planned for the region and began to tackle the last major hurdle -- deciding how much localities will pay to subsidize the operation of the train.

"That will be a tough nut to crack," state Del. David G. Brickley (D-Prince William) said after a meeting on the proposed one-year rail experiment. Brickley said he was optimistic a formula could be agreed on in time to start the trains on Jan. 12.

Rep. Stan Parris (R-Va.), a major supporter of the project, said planners estimate the maximum fare will be $6 round trip from Fredericksburg, Va., to Union Station in Washington. Fares for shorter trips will be less.

The train is scheduled to leave Fredericksburg at 7:05 a.m. daily and arrive at Union Station at 8:20 a.m. In the evening, it would leave Washington at 5:50 p.m. and arrive in Fredericksburg at 7:05 p.m.

The train also is scheduled to stop at L'Enfant Plaza, Crystal City, Alexandria, Springfield, Lorton, Woodbridge, Quantico and Brooke.

The Urban Mass Transportation Administration agreed earlier this week to pay 75 percent of the cost of building platforms and parking lots at the stops. Planners estimate the federal share will be $750,000 to $1 million. Brickley added that the state of Virginia will contribute a "substantial" amount of the remaining 25 percent, probably $200,000 to $300,000.

Transit planners expect fares to generate $400,000 during the first year, against projected costs of $1.2 million. Local governments are expected to pay the difference.

Fairfax officials have said they will refuse to bear the brunt of the costs and argue the assessment should be based on such factors as ridership. Other localities note, however, that Fairfax will benefit if the train eases the county's traffic problems.