After the 1982 Metro derailment at the Smithsonian station in which three passengers were killed and 25 were injured, federal safety experts urged the transit agency to provide its trains with "black box" recorders similar to those used aboard commercial airliners.

A long and costly process of study, design and evaluation is still in progress.

The electronic monitors, among a number of equipment and procedural improvements recommended by the National Transportation Safety Board, would record all operational data, such as speed, propulsion, braking, train control signals and door movements.

Voice tapes are made automatically of all operator conversations. But Metro trains move primarily according to automatic electronic signals. The board concluded that a record of those commands and the train's responses would be useful in accident investigations to reconstruct events, just as the tapes preserved by the "black boxes" on all commercial aircraft are used.

Three years ago, Metro received a $200,000 federal grant to design and build a prototype monitor to be tested on its trains.

Metro has since spent $30,000 on research to develop initial design requirements, or "specifications," for a prototype monitor.

The Metro board learned in September that it would lose the remaining $170,000 of the grant if it did not proceed further. So the board voted to move ahead with the project and is preparing to negotiate a contract for the design, construction and testing of the device, a process that could take up to two years, said Fady P. Bassily, Metro's assistant general manager for rail operations.

After testing the monitor, the Metro board will have to weigh its costs and benefits before deciding whether to install the device on its cars. The cost of equipping the cars could reach $10 million, according to preliminary estimates, Bassily said.

Other transit systems do not have such equipment, but at least two transit agencies in Canada have studied the idea, Bassily said.