After a seven-month probe, Metro officials said yesterday that they cannot explain why key electronic devices along the subway system have been failing at an alarming rate, so they will rip out the 20,000 devices and replace them with new ones.

Replacing the mechanisms, known as relays, will take about seven months and will allow Metro to resume operating trains by computer, officials said. That will mean a return by May of smoother, faster rides and less wear on brakes, they said.

"This is in the best interest, from the safety point of view," Metro General Manager Richard A. White said yesterday. "We've been looking for seven months for an answer to that problem, and we could have kept on looking. But it wasn't clear that we were going to get any more information than we have today."

The subway cars, designed to be run by computer, have been controlled by drivers since April, when the electronic failures reached a crisis point.

The relays, each one the size of a hardcover book, transmit information among trains, signals and switches and are key to preventing collisions. Located throughout the 95-mile system, the relays control switches for changing tracks; adjust each train's speed and braking to ensure adequate spacing between trains; control signals that tell operators whether to stop or proceed; and prevent two trains from occupying the same stretch of rail.

The relays, installed when Metro was built in the 1970s, were expected to last about 70 years. But four relays have failed in the past two years, sending erroneous instructions to various trains. In December, a train was told to go 45 mph on a stretch of track with a 15 mph speed limit. In February, a train was directed to travel at 0 mph when it should have been ordered to move at 15 mph. And in March, a train was halted when it got a red, or stop, signal, but the rail ahead was clear, and it should have received a green signal to proceed.

Statistically, the system should have no more than one relay malfunction every 50 years, transit officials said.

Months of analysis failed to determine the cause of the malfunctions. Metro hired consultants, including metallurgists, to study the relay problems, which included the presence of a salt-like substance on the metal parts. While it was obvious that the equipment was crashing, it was not clear whether it was the fault of Metro or the manufacturer, Alstom Signaling Inc., Metro officials said.

"In the end, we couldn't find a smoking gun," White said.

Metro agreed to buy 17,400 new relays from Alstom at lower-than-wholesale cost, but neither side would disclose the price or the warranty agreement, saying the terms were confidential. However, a transit source said the deal cost Metro less than $8 million and included a 40-year warranty.

White defended the decision to strike a deal with Alstom rather than try to sue the manufacturer for no-cost replacement. "This is an extremely good settlement for us--we got favorable pricing, extended warranty and accelerated delivery," White said. "Our system is automatic, our culture is an automatic culture. The system is better off, the passenger will be better off, and all of us will be better off when this is behind us."

A return to automated train operation should bring relief from another problem plaguing Metro subway cars--finicky brakes. Transit officials said yesterday that it will be 2001 before there are significant improvements to the braking systems of nearly 300 Metro cars under a $96 million emergency repair program launched in April.

The repair program was started after a series of breakdowns during the week of the Cherry Blossom Festival--a period of heavy Metro use--infuriated passengers and embarrassed Metro managers. The program calls for purchasing new rail cars and correcting brake problems among the 300 oldest cars in Metro's 590-car fleet. Braking problems are believed to cause about 60 percent of the system's delays and breakdowns.

Officials are reviewing three bids for the repair work and expect to choose one by December.

CAPTION: Metro mechanic Kenny Anderson works on a rail car at the Brentwood rail yard in D.C.