Here comes the mystery subway, pulling in at Metro Center. Take a guess. Where do you think it's going? One sign is bright orange and marked "New Carrollton." Another is blue and says "Addison Road." Still another warns "No Passengers." Look again. Would you believe . . . "Vienna"?

"You can see anything from 'Vienna' to 'Franconia' and 'Springfield,' " said Fairfax County Supervisor Joseph Alexander, a veteran sign watcher and chairman of Metro's operations committee. He also recalled seeing "Falls Church" on an Orange Line train to Ballston. "That's fine, but you don't go there."

After years of mixups over mismatched markers, Metro has come up with an answer: Junk the trouble-plagued signs on 298 subway cars and replace them with "proven, electronic, low-maintenance" contraptions. The Metro board voted yesterday to spend $3 million in federal funds to carry out the overhaul.

The malfunctioning signs have been "a nagging problem for everybody," said Fady P. Bassily, Metro's assistant general manager for rail service. "There was a basic design problem."

Two years ago, the transit authority spent $200,000 in the latest of several attempts to make the signs say what they should. But the scheme proved a dud. Once again, said Bassily, "it was the same problem."

Officials plan to start installing the new signs next fall and complete the work within a year. Federal assent appears assured, according to federal and Metro officials, because federal money already has been appropriated to improve the rail system's reliability.

Will Metro's new strategy pan out? "It should," said Bassily. "God, I sure hope so. We're counting on them working," added Metro spokesman Beverly R. Silverberg. After all, officials said, electronic signs were installed in Metro's new Italian-made subway cars and these seem to have worked properly.

The signs on Metro's 298 older subway cars have suffered an assortment of apparently intractable ailments. Their plastic scrolls jam. Pinpoint holes in the plastic let in light that triggers sensors to give erroneous signals. Damaged codes are electronically misread.

The result is faulty destination markers, including stations that have not opened or are not yet built, such as Vienna and Franconia-Springfield.

At the Pentagon, a passenger may mistake a Blue for a Yellow Line train. "For strangers in the system, it's awful confusing," said Alexander.

In other developments yesterday, the Metro board approved changes in bus routes in Northern Virginia and Prince George's County.

The Virginia changes, set to take effect March 31, were designed to increase bus service to Fairfax County's Huntington rail station and to cut costs by reducing service on routes that parallel the rail line.

All Route 9 buses are to stop at Huntington. Route 11 service from Fairfax to the Pentagon, National Airport and the downtown area is to be reduced. The 27H, 27X and 27W from Fairfax to the Pentagon are to be eliminated.

In Prince George's, a new Route 89 bus line is to start service Jan. 28 between Laurel and the Rhode Island Avenue rail station as an experiment lasting up to one year.