Proposals for starting a commuter rail system to serve outlying suburban areas in Northern Virginia have set off renewed debate as a result of a study released by the Metropolitan Washington Council of Governments.

A Northern Virginia system, similar to Maryland's commuter rail service, might help ease traffic congestion in the I-95 corridor, some officials said. A staff memo by the Northern Virginia Transportation Commission said the idea "seems promising."

The proposed system would include two branches linking Alexandria's Union Station with Fredericksburg and Manassas.

Special rush-hour trains would run on existing freight and passenger railroad routes to serve commuters from Fairfax, Prince William, Stafford, Spotsylvania and possibly Fauquier counties.

The $23,500 consultant's study concluded that a commuter rail system is "feasible," but it said the outlook is uncertain because of multimillion-dollar costs and "the lack of cohesive public initiative in support of commuter rail." Some railroad officials also objected to the plan.

The 141-page report, prepared by R.L. Banks & Associates Inc. for the Council of Governments and the Virginia Department of Highways and Transportation, represented the latest review of a long-simmering issue. Ten years ago a study also found the plan feasible, but said it faced obstacles.

Cost is a key issue in the current debate. The consultant's report said that setting up the proposed rail service would cost $46 million, and it estimated that the system would incur an annual $2 million deficit.

These estimates were questioned by Northern Virginia Transportation Commission officials. They argued that the costs could be sharply reduced by buying used, instead of new, rail cars and by taking other steps.

The overall cost of purchasing used rail cars "may be as low as $3.6 million," the commission's staff said in a recent memo. Operating costs also may be held down, the memo said, and expenses might be offset by as much as $4 million a year in additional federal aid under existing formulas.

"At $46 million, it's not even worth considering," said Richard K. Taube, the commission's executive director. "We think that the whole thing could be done for $3 million."

In Maryland, commuter trains operate on two routes connecting Washington's Union Station with Baltimore and Brunswick, about 40 miles northwest of the District. Stops include Bowie, Odenton, Silver Spring and Rockville. The service, used by more than 6,000 passengers daily, runs at about a $4 million-a-year deficit, the study said.

The proposed Northern Virginia system would likely attract more than 3,000 riders a day, according to Council of Governments estimates. One-way fares would range from $1 to $2.10. Under the proposal, trains would run every 30 minutes during morning and afternoon rush hours.

The Manassas line would use the existing Southern Railway Co. route, and it could be extended to either Gainesville or Calverton, according to the report.

The Fredericksburg line would follow the existing Richmond, Fredericksburg and Potomac Railroad Co. route.