An unexplained, hour-long power loss on one track at Metro's Federal Triangle station snarled the subway system's Blue and Orange lines during the evening rush hour yesterday and delayed thousands of riders.

Because one track was out of commission, Metro was forced to alternate one-way train operations between the Smithsonian and Farragut West stations. The result was that people sometimes waited up to 20 minutes for the right train.

Because of the delays, the platforms at most of Metro's downtown stations became jammed and Metro police had to control the entrances to both the stations and the platforms to reduce the dangers of overcrowding.

The incident began at the worst possible time, 3:57 p.m., when automatic circuit breakers cut the power to the inbound track at Federal Triangle. A train containing malfunctioning cars was being moved without passengers through the station at that time.

It took Metro electricians 57 minutes to restore power and get the stricken train out of the station. By that time it was almost 5 p.m., the heart of the rush hour, and the system was a shambles.

Many trains were located in the wrong places and were running in the wrong sequence. It was after 6 p.m. before Metro finally had most downtown platforms cleared.

Metro spokesman Cody Pfanstiehl said that hundreds of buses at such terminals as New Carrollton and the Pentagon were held so commuters could make connections. Many commuters doubtless missed their regular buses, however, Pfanstiehl said.

The cause of the power failure was under investigation last night.