Streetcars would shuttle 30,000 riders a day through the busy Route 1 corridor between Alexandria and the Pentagon under a proposal announced yesterday by Alexandria and Arlington officials.

The trolley line, which would cost about $400 million, would run along either Route 1 or a yet-to-be- built street in Potomac Yard, which is under development. The line would then connect to existing streets in Crystal City and in Alexandria, where it would end at the Braddock Road Metro Station.

"Let's take a bold step," said Alexandria City Council member Lois L. Walker (D), who announced the plan with Arlington County Board member Chris Zimmerman (D). "Let's see if we can bring a new image to parts of our community."

Zimmerman and Walker said that although no funding exists to build or operate the trolley, it is essential to easing traffic in the rapidly developing area. There is a parallel Metro line along the Route 1 corridor, but it offers relatively little service, with only one stop between Crystal City and Alexandria's Braddock Road station, and that is solely to serve Reagan National Airport.

The trolley would have about 10 stops, providing service to an area whose transportation needs are about to multiply. Commonwealth Atlantic Properties has plans to include about 10 million square feet of commercial and residential development in its Potomac Yard project in the old rail yard just south of the airport. That would bring at least 5,000 residents and 25,000 daily workers to the area within the next 20 years.

Although details of the plan are sketchy, the proposal calls for running electric-powered trolleys along rails in the middle of street or in the right-hand lanes. The trolleys, powered from overhead cables or ground-level rails, would stop at designated spots as buses do, but would provide a smoother ride, with no diesel fumes. Metro would operate the service.

The trolley proposal took shape after plans for a privately financed Metro station in Potomac Yard was dropped in the new project design. Alexandria officials have been loath to resurrect the idea, in part because of its $50 million price tag.

Even if a station were built in Alexandria's portion of Potomac Yard, however, it would be too far south to serve Arlington riders, Zimmerman said.

Funding is the biggest stumbling block for the trolley proposal, which was first floated as part of a regional $11 billion package of transit and road projects designed to untangle Northern Virginia's traffic jams. Officials hope for federal and state funding, as well as local public and private money.

For instance, Potomac Yard's developer might be asked to provide right of way for the trolley, officials suggested. Commercial property owners also could agree to form a special tax district to raise funds for construction and operation.

Such a special tax district could raise about $1.2 million annually in Alexandria, according to Mark Jinks, the city's finance director. Project supporters provided no projections on annual operating costs, but Zimmerman said fares would be expected to provide about 50 percent of those costs.

Officials said the still-vacant land of Potomac Yard is the ideal place to plan future transportation routes, and that light rail has worked well in many U.S. cities, including Portland, Ore., and Dallas.

"We have an opportunity to look ahead with something that isn't developed," Zimmerman said. "People hate buses, but they love trains."