Thousands of rush-hour commuters poured out of their buses to change to subways at the Rosslyn Metro Station yesterday morning only to find that the trains had quit.

It took Metro one crucial hour - from 7 to 8 o'clock - to solve the problem: two mis-set control panel switches deep in the Rosslyn Station. Why those switches were in the wrong position and why they did not stop earlier trains are matters under investigation.

The result was utler confusion for commuters who were put through a major bus realignment Aug. 1 and forced to take the train whether they wanted to or not. Primarily for financial reasons. Metro on that date curtailed many commuter bus routes at outlying subway stations. While the subway was down yesterday. Metro attempted to bridge the missing link with reserve buses.

Since Metro's Blue Line from National Airport to RFK Stadium opened July 1, the system has been troubled with persistent mechanical difficulties; doors stick; brakes lock, signals don't always work. Passengers have noticed other things, the automated fare collecting equipment initially confusing and there isn't enough of it at some stations, frequently escalators are not running; destination signs on the train indicate they are going to nonexistent destination signs on the trains indistops such as "Dulles Airport."

A study of Metro's train performance shows that since the Blue Line opened on July 1, trains have made only 83.4 per cent of their scheduled runs. There never has been a 100 per cent day; the best day to date was last Thursday when 213 of 218 scheduled trips (97.7 per cent) were completed.

Although performance has been improving steadily, yesterday mornings was the worst delay to date.

"They let us off the bus in front of the Rosslyn station," Metro rider Greta Meenehan said. "The bus pulled away; everybody ran up to the station like they always do, but Metro people were there waving their arms and saying we couldn't use the train. It's funny now, but we all sort of milled around, looking at each other."

"It's not funny either," a coworker of Meenehan's at the Veterans Administration said. "I missed an appointment."

Meenehan and three friends caught a cab from Washington while Metro's bus dispatchers were trying to round up reserve buses and send those they could into the District.

Meanwhile, Metro's train controllers tried to run what they could of a railroad. Eight of 13 Blue Line trains were still on the Washington side of the Potomac River, and they began to shuttle between Foggy Bottom Station and the end of the Blue Line at Stadium-Armory. There were some delays on the Washington-only segment, but a semblance of normal service was maintained.

On the Virginia side it was a shambles. There were stalled trains on each track at Rosslyn and three other trains in Virginia. Metro began running those three between National Airport and the Pentagon. From the Pentagon, good bus service to Washington still is available.

Why the trains stalled is somewhat curious. About 6:50 a.m., trains stopped on both tracks at Rosslyn and refused to move, Normally, such problems are caused by equipment on the trains, such as sticking doors or brakes.

In this case, the culprit was in the automatic train control system. A cable was in the automatic train control system. A cable runs along Metro's tracks and transmits information to each train. Signals tell the train when to brake, when to apply power, when to open the doors, how long to stand at a platform. Theoretically, the trains could run without operators.

Train control rooms are located in each of the Metro stations. They contain the sophisticated equipment that feeds the information to trains for a given section of track.

One of the pieces of information that the room transmit is called a "speed readout." The train operator's console contains three digitized blocks. One lists the speed limit for the section of track the train occupies; the other lists the speed allowed under present operating conditions (for example, if the train is closely following another train the speed limit might be 65 miles per hour but the speed allowed would be 45 miles per hour) and the third is a simple speedometer. If the speedometer reading exceeds the allowed speed, the train automatically brakes.

When the train is standing in the station, the regulated speed is listed as zero. As long as the zero appears on the dial, it theoretically is impossible to move the train via automatic control.

The two trains stopped at Rosslyn were given speed commands of zero and the commands were never rescinded. As a result the trains stood there.

A technician was brought in from nearby Arlington Cemetery station to figure out what happened. He found that two switches (in the train control room were set so that trains were commanded to stand in the station with their doors open. Forever.

Metro officials theorized that the switches had been left in that position by a signal-testing crew that had worked on the tracks overnight. The problem with that theory is that 13 trains went through the Rosslyn station yesterday morning before the breakdown occurred. How did that happen?

"I'm waiting for that answer, too, and I do not have it," Metro general manager Theodore Lutz said yesterday. Metro officials were interviewing other train operators and checking train control tapes to see if they find the answer.

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Mrs. Meenehan said that her cab ride to the District have faith in Metro. However said, she and other are talking [WORD ILLEGIBLE] a car pool. Then we won't have hassle."