The water receded yesterday from a flooded 2,000-foot section of Metro's Blue Line in Southwest Washington, but whether full subway services can be restored by Monday was still uncertain.

Metro officials said crews would work around the clock replacing electrical switches and circuits which had been recovered by water from the Potomac River on Thursday evening when a barrier gave way at a construction site.

Metro general manager Theodore C. Lutz said the transit authority hoped to be able to run a test train late SUnday afternoon over the tracks that had been under water.

If the test goes well, he said, full service will be restored. However, if the trains can't run, Lutz promised that Metro will provide free shuttle bus service on the route of the seven stations - from Stadium-Armory to the Smithsonian - which had been closed because of flooding.

"The key to our decision affecting the trains on Monday," Lutz told reporters late yesterday afternoon, "is that we feel it is imperative to run actual trains through there before serving the public."

Metro community relations director Cody Pfanstiehi said it was "quite possible" that service would be restored to all Blue Line stations Monday morning, and "very, very possible" that the whole line will be operating Monday afternoon.

The entire subway system has been closed, as usual, on Saturday and Sunday.

By 6 p.m. yesterday the Blue Line tracks at the L'Enfant Plaza station were dry. Pfanstiehi reported there still was more than a half foot of water at the Federal Center-Southwest station, the low point on the line, where four pumps were operating. But he said he expected the tracks there to be dry about midnight.

He said workmen began replacing wet switches and cables in early afternoon shortly after water stopped dripping onto the Blue Line tracks at L'Enfant Plaza from a tunnel for Metro's uncompleted Yellow Line which crosses above it there.

Water started entering the subway system about 7 p.m. Thursday through a break in a steel-plate barrier called acofferdam, on the bank of the Potomac River near 9th Street SW and Maine Avenue. Within 15 minutes the water broke through the other nearby barriers, and then poured through a half-mile tunnel of the unfinished Yellow Line.

It reached the L'Enfant Plaza station, at 7th and C Street SW, about four hours later, and flowed downgrade on the Blue Line to the station at Federal Center Station were almost five feet deep.

Although water was still seeping in from the Potomac late yesterday, the flow had been sharply reduced by rebuilding one of the broken barriers with sand and gravel.

Workmen also built a three-foot high sandbag barrier in the Yellow Line tunnel, about 20 yards before it reaches the L'Enfant Plaza station. Late yesterday afternoon the water behind the sandbags was about two feet high and receding, as pumps at the riverfront sent water back into Washington Channel.

Pfanstiehl said Metro contractors planned to construct a giant concrete plug within the Yellow Line tunnel, 16 feet in diameter and 42 inches thick. He said the plug would be set in place late today or Monday several hundred feet before the tunnel reaches the L'Enfant station so that no water can get to the Blue Line no matter what happens along the riverfront.

The accident Thursday night occured where the Yellow Line tunnel is to join two sunken concrete tubes that will carry trains about 900 feet across the bottom of Washington Channel. The line then crosses the Potomac on a new bridge and joins with the Blue Line subway again at the Pentagon.

Eventually, the line is planned to run up the Seventh Street corridor to College Park and Greenbelt, but no funds have been appreciated to extend it beyond the Gallery Place station, 7th and G Streets NW.

Yesterday Jack Peterson, an engineer for Bechtel Associates, the firm that is supervising construction for Metro, said the failure of the three barriers was "a one in a million chance."

Extremely uncommon is the only way to describe it ," Peterson said. "We can't be sure if anyone is at fault until we know what caused it."

He said a diver inspected one of the damaged barriers yesterday morning, and found its walls were still in place, suggesting that a crack may have developed in the concrete floor of the barrier allowing water to enter.