Maryland and Virginia have not figured out what to do with two of the 12 lanes planned for the new Woodrow Wilson Bridge now rising in the Potomac River.

The lanes must be used for carpools, trains or another form of transit under the deal that launched the project to replace the aging span, but state leaders said they have not talked about which to choose.

"When the bridge opens, those lanes at that time will represent excess capacity," Maryland Transportation Secretary Robert L. Flanagan said.

The lanes will serve as shoulders until a use is designated.

In an indication of how little the two states have discussed the issue, Maryland leaders said they're not considering rail because Virginia opposes it, while Virginia's transportation secretary said the state is open to rail and other possibilities.

Planners said the situation will have little direct effect on drivers because the new, twin-span bridge scheduled to be completed in 2008 will have as many lanes as there are on the Capital Beltway segments it will link.

But the delay in deliberation has frustrated transit supporters, who fought to add the lanes and ensure that the bridge was strong enough to sustain trains, the type of transit they would like to see. Their hope is that a light-rail or Metro line will be extended across the bridge and will become part of a circular line that mirrors the Beltway.

They said they are not as concerned about whether transit is on the bridge the moment it opens as they are about regional priorities. While talk of transit has lagged, officials in Virginia have inked a deal to add toll lanes to a portion of the Beltway, and Maryland officials are proceeding with plans to widen their side of the road.

"It just seems like nobody's really been willing to be the champion for this," said Chris Carney, conservation organizer for the Sierra Club. "It just seems like a lot of inaction."

Dru Schmidt-Perkins, executive director of 1000 Friends of Maryland, an environmental group, said that "it's as important to get future light rail or heavy rail across the bridge as it is to get truck traffic through" and that priorities are limited to "a lot of roads and an awful lot of pavement . . . that's just going to be filled up with congestion."

Metro drew up plans for a line four years ago, but officials said work has been set aside in the absence of a decision on the bridge.

Transportation officials in both states said they've had only the most preliminary of discussions about what to do with the lanes because they have been focusing on other priorities, including building an east-west highway in the Maryland suburbs and toll lanes on the Beltway. They said they plan to get together this year to begin formal talks.

When fully complete, the $2.43 billion bridge will double the capacity of the old six-lane span. Three local lanes and three express lanes will be separated by a short barrier, in each direction. Of those, eight will be for general travel -- two express and two local in each direction -- to match the number on the Beltway. Additionally, the farthest right local lane on each span will serve as a merge for drivers using the interchanges near the bridge.

The first span is scheduled to open in May 2006 and the second, which will be built where the current one stands, in 2008.

Even after a decision on transit is made, officials and construction managers said it could be years before the lanes open because any option would require considerable cost and construction on both shores.

Some said this is no big deal -- the bridge is made to last 75 years, and it's not critical to have transit at its outset. "The good news is that they're there to put in place when the best use is determined," said Bob Chase, president of the Northern Virginia Transportation Alliance.

But others said the lack of progress threatens to put off rail for a good chunk of those 75 years, a view confirmed by officials who said that expanding Metro across the bridge would require considerable cost, construction and delay.

"A heavy-rail alternative would be many, many years down the road," Virginia Transportation Secretary Pierce R. Homer said.

Others said they worried that the delay would nix rail altogether as pressure builds to do something simpler, such as using the space for high-occupancy toll lanes. HOT lanes, the concept approved for parts of the Virginia Beltway, would satisfy the transit requirement because they are free for buses and carpools, while others would pay for the privilege of using them.

"I'm very worried that the HOT-lane decision was the first of the dominoes and forces and predetermines that a Wilson Bridge HOV-HOT would feed into that," said Stewart Schwartz, executive director of the Coalition for Smarter Growth. "Rail seems nowhere on the radar screen. I'm mad that they're not advancing rail now and that they didn't do it from the beginning of the project."

Planners said that even this option would require a minimum of seven years to complete because of various studies that would need to be done and because carpool lanes would require several more interchanges and an expanded Beltway to accommodate them on both sides of the bridge.

Flanagan said it was conceivable to open the lanes on the bridge for bus service or HOV before then, but "it would be an isolated HOV, not part of a system or overall traffic plan."

The Wilson Bridge span under construction, the first of two planned, is scheduled to open in the spring.