For three years, Metro's subway cars have carried destination signs that bore little relationship to fact. Trains going to Silver Spring might say Cheverly. Trains going to National Airport might say West Falls Church. On one memorable occasion, each car on an eight-car train bore a different sign. Riders could take their pick.

"You might say that fixing the destination signs has not been the highest priority item for us," said Erich Vogel, the man in charge of making Metro's subway cars work.

On Dec. 1, when Metro opens its Orange Line extension from Rosslyn to Ballston, that priority will change, Vogel said. If it does not, thousands of unhappy Metro customers seeking to go to the Pentagon, for example, may find themselves in Clarendon section of Arlington instead.

The reason is simple.

When the Ballston line opens, Metro will take another step toward the real world of big city subways. For the first time, trains sharing the same track for part of the distance will be headed for different terminal points. Some confusion, abetted by balky destination signs on the trains, is inevitable.

Today, the Orange and Blue lines run as if they were one, all the way from New Carrollton to National Airport. That is a matter of operating convenience and will end with the opening of Ballston and three other stations on the new Orange Line segment.

After Dec. 1, Orange Line trains will run between New Carrollton and Ballston and Blue Line trains will run between Stadium-Armory and National airport. They will share the track between Rosslyn and Stadium Armory.

This means:

Riders desiring to take trains for any station between Rosslyn and Stadium-Armory can board either Orange or Blue line trains that are headed in the right direction.

Those desiring to go to Minnesota Avenue, Deanwood, Cheverly, Landover and New Carrollton must do so on a train that says New Carrollton. The trains marked Stadium-Armory will unload there, then turn back for their run to National Airport.

Those desiring to go to Arlington Cemetery, Pentagon, Pentagon City, Crystal City and National Airport myust do so on a train that says National Airport.

Those desiring to go to Courthouse, Clarendon, Virginia Square or Ballston -- the new stations opening in Arlington -- must do so on a train that says Ballston.

After Dec. 1, Blue Line trains should bear signs with blue backgrounds that say either National Airport or Stadium-Armory. Orange Line trains should bear signs with orange backgrounds that say either New Carrollton or Ballston.

On the sections of common track -- between Rosslyn and Stadium Armory -- every other train is supposed to be a Blue Line train -- or an Orange Line, depending on when you start counting. b

During rush hours, trains on both the Blue and Orange lines are scheduled to run every six minutes. On the common track between Stadium-Armory and Rosslyn, trains will run every three minutes. During the rest of the day, trains will run every 12 minutes on the Blue and Orange lines, every six minutes on the sections of common track.

That brings us back to the destination signs, and why they don't always work very well.

Metro's first 300 subway cars were built by Rohr Industries for about $300,000 each -- a price that now looks like an enormous bargain because the next shipment of cars will cost more than twice that much.

Included in each car are five destination signs -- two on each side and one on the front. The signs for each car -- including electrical circuitry and motors -- are estimated by Metro to cost about $3,000 each. So the signs for Metro's first 300 cars cost about $900,000.

The signs were manufactured by Teleweld, a Chicago firm and subcontractor to Rohr.

Each sign consists of a long sheet of plastic, much like a window shade, and lists about 5 possible destinations. Included are such wish-book destinations as Dulles Airport, where early Metro visionaries once planned to run a Metro line after the 101-mile system was completed -- which looks about 10 years away under the current schedule.

With 50 destinations, the curtains are bulky and difficult to control by motor. Furthermore, there have been problems in the electronic connections between the lead car of a train -- where the correct destination sign is selected either by the master Metro computer or by the train operator -- and the individual signs. Work has been done to alleviate that problem, but it's not as good as if it had been done right in the first place," Metro's Fady Bassily said.

Both Metro's Vogel and Teleweld president Chet McKee agree that maintenance of the signs has not been a priority item for Metro. In the past three years, Metro has concentrated on other difficulties, among them sticking doors and unscheduled brake applications. Metro's passengers quickly learned that if they stood on the right platform and took the next train they would get where they wanted to go no matter what the train sign said.

Teleweld's McKee, in a letter to The Washington Post, said that "With five signs in each car the complexity of the system is illustrated by the following partial list of components in one carset of signs: 10 gearmotors controlled by 10 power relays; 35 transistors related to the above power relays; 55 light-emitting diodes; 55 light-sensitive transistors; 55 reed relays; 40 integrated circuit units; several hundred resistors, capacitors, diodes, other transistors, etc."

In an effort to simplify the maintenance burden, Metro has decided that only one sign on each side of the car is necessary, which is why most trains now have one sign showing a destination and the other sign showing a blank. Furthermore, a new type of sign has been specified for the next order of subway cars that is now under design in Italy.

The new signs, according to Bassily,will not be as pretty, will cost more, but will work.