Thousands of Washington area subway riders were forced to drive or take unfamiliar bus routes to work during morning rush hour yesterday after an electronic computer switching device failed and left Metro's trains powerless for more than an hour.

The drivers encountered the kinds of traffic jams that are usually reserved for snowstorms and the bus riders found themselves temporarily stranded at subway stations until infrequently scheduled downtown-bound buses would come along, usually full.

"It's stupid," said James McQueen of Southeast Washington, who was deposited by one bus at the Potomac Avenue subway station at 7:15 a.m. and had to wait 20 minutes before he could find a seat on a downtown-bound bus. "Why did they drop everybody off instead of just bringing them downtown?"

Metro was able to add about 25 buses to the 1,800 that normally run on a weekday morning. "But there's no way we can duplicate the subway on that short notice," said Homer Cogdill, Metro's senior street bus supervisor.

Metro was able to store full subway service by 7:45 a.m., on hour and 45 minutes late. By that time thousands of people had already been inconvenienced by trains that did not run or had heard about the problem on the radio and had elected to drive to work. About 80,000 to 85,000 people use the subway during an average weekday morning rush hour.

One of them, Lori Dobbs, arrived at the Metro station at Silver Spring early yesterday and found it closed. "I started running around to all the buses because I didn't know how to get downtown by bus," she said. "Finally they opened the doors to the subway, and I waited until it ran."

Maggie Baker, who works at the Sheraton-Carlton Hotel downtown, had a similar experience at the Rhode Island Avenue station. With the trains not running, "I panicked," Baker said. "I saw a bunch of people with their thumbs out on Rhode Island Avenue. So I did the same thing, and got to work on time."

The evening rush hour yesterday went smoothly, although Metro reported that it was carrying fewer people than usual on both the subway and bus systems.

As is always the case in such situations, the people hardest hit were those who have to cross either the Anacostia or Potomac River bridges to get to work. The major Virginia arterials into Washington, especially Shirley Highway, were unusually crowded yesterday. There was still a substantial back-up at the 14th Street Bridge as late as 9:30 a.m., when traffic would usually have been flowing freely for 30 minutes.

Gary Wendt of the D.C. Transportation Department said an informal survey showed unusually heavy traffic on Georgia Avenue, Benning Road and 14th Street, among major roads. George Cook of Colonial Parking said that business appeared to be brisker than usual at downtown parking garages. Several people employed in the Connecticut Avenue area said they had trouble finding space for their cars.

Among those people dumped off their bus and into a long line of standees at the Ballston Metro station yesterday morning was Richard S. Page, Metro's general manager.

"Somebody recognized me," Page said later, "and asked, with some profanity, 'Why don't you idiots have a back-up bus system?' This line I was standing in contained at least six busfulls of people, it was cold, and I agreed with her that it was a good question."

Over the last three years, as Metro's subway lines have extended farther and farther out, hundreds of Metro bus routes that originally terminated downtown have been changed to end at outlying subway stations. That has meant some economies in the bus system and has permitted many buses to make more than one inbound trip during morning rush hour. If the subway isn't running, however, the system is flawed.

Yesterday, some buses with outlying subway stop destinations were permitted by their supervisors to continue into downtown and most of the 25 additional buses were used for downtown trips from such subway stations as the Pentagon, Ballston, New Carrollton and Silver Spring.

"It's better to let people pile up at the subway stations and let them transfer as they can than to leave them stranded back down the line," said supervisor Cogdill. Metro officials knew by 7 a.m. that they were going to be able to restore full subway service within the hour allowing those commuters still at the subway stops to take the rails to work.

Cogdill's headache began at 4:25 a.m. when a technician in Metro's control room threw a switch and nothing happened.

The switch is on an electronic box called a multiplex. It is supposed to send signals from Metro's control center in the basement of the headquarters building to Metro's master computer in an adjacent room.

At that moment, the signals needed were the ones that would turn on third-rail track power to several sections of both Red and Blue/Orange lines, where workmen had been doing maintenance earlier. For safety reasons, the 750-volt-third rail had been turned off.

The technician tried the switch again. Still nothing. The television screens in Metro's control room were blank. That meant that track switches could not be turned automatically, power could not be restored to the third rail and that the progress of trains could not be monitored.

The railroad could be placed in operation, however, if all those functions were perfomed by hand. Crews were dispatched to restore power to the various sections of the subway. They had to visit 12 different buildings throughout the system to do that.

By 7 a.m. most of the power was restored. Work trains carrying station attendents were dispatched and the system was slowly opened. Technicians at Metro headquarters got the mulitplex back in business by 8 a.m. An investigation was under way last night to determine exactly what had gone wrong.

There was at least one happy Metro customer yesterday morning. Elizabeth Ann Taylor of Crofton parked her car in the New Carrollton lot at 6:50 a.m. She walked to the station, to be told by supervisors that the train was not running and which bus to board. A jam-packed bus then proceeded to Potomac Avenue, where passengers were supposed to transfer to another bus.

"The bus driver opened the door and told his supervisor we were all going to work (downtown) and asked if he could continue. The supervisor nodded. After the driver changed the destination [sign] on the bus, we all gave him three hip-hip-hoorays. I was only 20 minutes late for work."