Commuter rail service linking the outer Virginia suburbs with the District of Columbia inched closer this week after winning a federal grant, but it continues to face several serious obstacles, proponents said yesterday.

The U.S. Urban Mass Transportation Administration announced plans to award $750,000 in federal money to pay most of the costs of constructing parking lots and station platforms along the proposed line connecting Washington's Union Station with Fredericksburg and Manassas.

The grant, while a small portion of the more than $40 million in start-up costs, is "an important part of the total," said Richard K. Taube, executive director of the Northern Virginia Transportation Commission, the regional agency working to start the service. "It signals that the federal government recognizes this is an important project for the region."

The service could begin as soon as March 1989 if planners can resolve several serious issues, including the problem of obtaining insurance and the opposition of one railroad to allowing commuter trains on its tracks, Taube said. That opposition was reiterated yesterday.

Commuter rail organizers envision running four rush-hour round trips a day along two corridors to Washington. Planners say that the service would carry from 3,500 to more than 4,000 commuters inbound each weekday morning, depending on parking constraints, and the same number home each evening.

The plan calls for Amtrak to operate and maintain the trains, which would run over track owned by Amtrak and three other railroads.

The railroads have said they will not participate in the project unless they are protected from liability in the event of an accident.

Commuter rail organizers hope to satisfy those demands by arranging for the commuter rail service, with state assistance, to insure itself for the first $5 million in liability per accident and to buy additional insurance at an undetermined cost.

Commuter rail organizers have been negotiating for three years to obtain rights to use the track owned by Amtrak, Norfolk Southern Corp., the Richmond, Fredericksburg & Potomac Railroad Co. and Consolidated Rail Corp. (Conrail).

The project met an unexpected obstacle in June when Conrail announced that it would not permit the trains on its track. Conrail, the freight railroad recently sold to investors by the federal government, owns the 1 1/2 miles of track between the Potomac River and Virginia Avenue in the District, including the site of a proposed L'Enfant Plaza commuter rail stop.

Without Conrail's track, the commuter trains could not reach Washington Union Station and would have to terminate at Crystal City or at Alexandria's Union Station, near the King Street Metro station.

Conrail officials said the two additional commuter trains a day would disrupt its train schedules.

Commuter rail proponents, including local politicians, have hoped that Conrail officials would change their minds.

But Fairfax County Board Chairman John F. Herrity said yesterday that he recently received a letter from Conrail Chairman L. Stanley Crane repeating the railroad's position.

"This letter is the death knell of commuter rail, unless you want to stop at Alexandria and hook up with Metro," said Herrity, who plans to meet with Crane Sept. 25.Staff writer John Lancaster contributed to this report.