Metro's Yellow Line, on which construction was disrupted by flooding yesterday, has been clouded by uncertainly for several years.

At its southern end, the line is supposed to extend to the Franconia section of Fairfax CountY. In the District of Columbia, it is supposed to go through the city's poorest areas, Shaw and Cardozo. In Prince George's County, it is to reach colege Park and, Greenbelt and perhaps extend eventually to Laurel along the B&o Railroad.

Virginia and Maryland officials have been concerned about the cost of the yellow line. City officials have been uncertain about its alignment, which was switched several years ago. Instead of going straight out 7th Street NW and Georgia Avenue, the line bends near U Street and at 14th Street and Park Road. Then it returns to Georgia Avenue at Varnum Street NW.

Even though the people who live along much of the proposed line are the poorest in the area and presumably much in need of public transit, extension of the Yellow Line beyond 7th and G Streets NW is one of the last sections on Metro's construction schedule. Now because of soaring construction costs, officials fear it may never be built.

Whether it will be porbably depends on an "alternatives analysis" about Metro' by the federal Department of Transportation, the agency tht is footing most of Metro's construction bills. The study is supposed to be completed in several months.

No matter how it comes out, though, the Yellow Line's tunnel and bridge across the Potomac are going ahead. the contracts for them were let several years ago.

By cutting across Hains Point from the Pentagon to L'Enfant Plaza, the Yellow Line substantially shortens the trip to down-town Washington for Alexandria commuters. Otherwise, they would have to ride through nine extra stops on the Blue Line, subway which crosses the Potomac at Rossyln.

However, if the two ends of the Yellow Line never are built - to Franconia and Greenbelt - its middle section, which already has cost $38.9 million probably will have relatively few passengers.