Heavy metal brake discs fell off operating Metrorail cars seven times between July 1980 and August 1981, posing the potential for a serious accident, but Metrorail operations chief Joe Sheard said that no injuries or serious damage to cars occurred in the incidents and that careful testing and more frequent replacement of discs has cleared up the problem.

The last fall-off occurred on Aug. 5, Sheard said. Metro has ordered discs -- designed for heavier wear -- that eventually will replace all of the current stock, he said.

Subway operators in general are fearful of foreign objects on the tracks, because they could derail a train or cause short circuits in the electrified third rails, posing a fire threat to an immobilized train. Loss of a disc does not significantly affect braking capability, because there are seven other discs on each car, Sheard said.

The 135-pound, doughnut-shaped steel discs broke off, Sheard said, after developing tiny cracks. The loss in most cases was noticed at the next station or during routine maintenance, and crews were dispatched to take the broken parts off the tracks. No piece was ever found in contact with a third rail, Sheard said, and Metro never found it necessary to kill power in the rail to recover them.

The first fall-off occurred on July 25, 1980, Sheard said. The cause was identified near the end of the year, following detailed study of the breakage and another brake problem, deafening screeches that forced passengers to hold their ears. (The noise was eliminated by changing the brake pads. It is not clear whether conditions that created the noise had a direct role in cracking the discs).

To combat the disc problem, Sheard said, Metro is conducting "dye penetrant" tests on the discs every 30 days to detect tiny cracks invisible to the eye. In addition, Metro has reduced the amount of wear that each disc can receive before it routinely is replaced.

Before the problem was identified, Metro had begun a program to replace the discs with other types that would wear longer, Sheard said. About 10 percent of the discs on the 300-car fleet already have been replaced with stronger versions, and other cars are retrofitted as the new components arrive from the factory.

Brad Dunbar, spokesman for the National Transportation Safety Board, said board investigators knew of no similar incidents with breaking discs in other transit systems. They agreed, he said, that the separated discs could potentially derail trains or cause short-circuits and fire.

In other transit systems, debris on the track has been blamed for serious accidents.

One firefighter was killed and about 50 people injured in San Francisco's Bay Area Rapid Transit rail system in January 1979 after a train struck a switch box cover that had fallen from an earlier train. The box cover broke a "shoe" assembly that feeds electric power from the third rail into the train, causing a fire.

BART later reversed longstanding policies and through posters and brochures began informing riders how to escape from disabled trains. Metro's emergency plans, endorsed by many other transit officials, stress moving trains on to the next station by whatever means available, or, that failing, using operators or rescue workers to escort passengers to safety on foot.

Metro does not publicize the location of emergency switches and latches, on the grounds that doing so could lead to passengers jumping out onto lethally charged third rails or misuse by pranksters.

The National Transportation Safety Board and some area fire chiefs have criticized Metro for those emergency plans, calling them insufficient preparation for rail accidents. Metro officials respond that its moving rails cars have never been responsible for a passenger injury. There has been not one collision, derailment or fire since the system's first track opened in 1976, they say.