Metro activated its "silent alarm" system for bus drivers yesterday, a week and a half after the rape of a driver triggered a city-wide walkout over bus problems.

The alarm system, in the works for about three years, enables drivers to signal for help in case of emergencies to metro headquarters at Fifth and F streets NW. There a computer will identify the bus sending the signal, determine which route that bus is handling on a given day, and calcualte where the bus, if on schedule, should be. Police can then be sent to that location to give assistance.

The silent alarm is part of a $5 million two-way radio system that has been in service on the city's 1,800 buses for about six months, according to Metro spokesman Cody Pfanstiehl.

The silent alarms were installed in most buses a year ago, said Pfanstiehl, but it has taken the intervening time to program the headquarters computer unit.

"You have to punch 85,000 punch cards to load the computer," Pfanstiehl explained, adding that there are 750 bus routes in Washington and as many as 1,650 buses in service at one time. Every morning, the computer will have to be updated with as many as 1,650 pieces of new information - namely, which bus has which route.

Although rapid completion of the silent alarm system was part of a safety agreement worked out the week before last between Metro and the city's bus drivers, Pfanstiehl said it was merely "coincidence" that the system went into operation when it did.

The original target date had been May 19, he said, but computer problems had delayed the operation until yesterday.

A female bus driver was raped a week ago Wednesday while her bus was parked at Texas Avenue and Ridge Road SE, a bus route layover point. Her assailant forced the woman from the bus and into nearby Fort Dupont Park, where the rape occured, according to police.

Bus drivers in Southeast walked off their jobs the following day, and the walkout spread throughout the city, with union officials complaining about what they said was grossly inadequate security - particularly when compared to protection in the subway.

The deal subsequently struck in three-way talks involving Metro, its drivers and the city government, called for stepped-up policing of buses, including use of plainclothes police officers in addition to completion of the silent alarm system.