A section of 750-volt "third rail" between the Metro Center and Federal Triangle stations was short-circuited by a loose scrap of aluminum yesterday afternoon as a train passed by, causing brief loss of power and heavy smoke, Metro reported.

Third rail incidents of this sort are extremely rare and have contributed to accidents in other subway systems. "Anything falling on the right-of-way is certainly reason for serious concern," said Harold Barley, Metrorail operations officer.

No one was injured in the incident, which tied up Blue and Orange Line traffic for more than 30 minutes when technicians closed off the track and searched for the cause of the short circuit.

Metro officials believe that a small piece of "cladding", thin aluminum leaf which holds pipe insulation in place, came free somehow and was blown under the third rail's protective cover when a Blue Line train carrying passengers to Addison Road passed by at about 1:20 p.m.

The short circuit tripped safety devices that cut off power automatically, causing the train to slow momentarily. The electrical energy disintegrated the aluminum scrap and the devices turned power back on within seconds, again automatically. Technicians later shut off the power manually to inspect the track.

Barley said that aluminum cladding had never before caused such a short circuit. He called it an isolated occurrence that did not seem to reflect basic design flaws.

In an unrelated incident yesterday morning, thousands of Metrorail commuters were late to work after a malfunctioning engine crippled a train and filled Farragut West station with smoke, forcing its closure for about 40 minutes.

With air conditioners shut off to avoid drawing in fumes, other trains passed through the station without stopping, using the unblocked track. "There were delays of 30 minutes or more to some passengers in some parts of the Blue/Orange Line," Metro spokesman Cody Pfanstiehl said.