Metro directors ordered a rail guard installed on a section of track at the West Falls Church Station after learning from the transit agency's top safety official yesterday that a Blue Line train derailed last month in part because it ran on rail that lacked a guard and was not designed for everyday use.

"It wouldn't have happened if the rail was guarded," said Katherine K. Hanley, who represents Fairfax County on the Metro board.

West Falls Church Station includes one of at least 10 areas of Metro track with unguarded rail, but it is the only stretch where passenger trains routinely run.

A rail guard is a metal lip that prevents the wheel of a car from leaving the track. Metro safety standards require rail guards on sharp curves along its tracks. The stretch of track north of the Reagan National Airport Station where the six-car Blue Line train derailed would fit that category. But it lacked a rail guard because it was designed as a storage track and not for daily use.

In November, Metro managers routed daily trains on the storage track, apparently unaware that it was not designed for that use, according to Fred Goodine, the Metro assistant general manager who oversaw an investigation into the Jan. 20 derailment.

Goodine described his findings to the board of directors at a public meeting yesterday. Metro Chief Executive Officer Richard A. White wanted the briefing to be private, saying the investigation was incomplete. But several board members insisted it be made public, after the findings were reported in yesterday's Washington Post.

"We wanted to be upfront with folks," said T. Dana Kaufmann, another Fairfax County representative on the board. "This is a necessary session."

Goodine, explaining the probable causes of the derailment, said the train was running on rail that didn't meet Metro's standards. Maintenance work performed earlier that day pulled the rails closer together, made its curve sharper and increased the chances for a derailment, he said.

The Jan. 20 derailment took place about 8:20 p.m., moments after the train left the platform at National, headed toward Crystal City. The train was traveling on a stretch of track that was designed to store cars temporarily but that had been used for daily service since November because a construction project had made the regular inbound track, Track 1, unavailable.

Metro officials didn't check to see whether the center track was engineered to handle routine traffic -- trains running every three minutes -- before they pressed it into service, Goodine said. If the track had been designed for daily use, Metro's safety standards would have required a rail guard. The track had such a guard only on a short section near a switch.

Metro had used the storage track for regular service without a problem in the past, giving subway managers confidence that they could do so without trouble, Goodine said.

Shortly after the daily service began, at least one track supervisor worried about the safety of the operation, but his concerns went unheeded, Goodine said.

The investigation unearthed other problems, including the way Metro responded to the accident. When the operator of Train 410 told Metro headquarters his train had jumped the tracks, the train controller suggested that the operator restart the train and try to move it to the next station -- a directive that violated Metro rules and could have damaged the train and injured riders, Goodine said.

"It would have made a bad situation worse," he said.

The train operator was not able to restart the train. Approximately 60 passengers on board had to be evacuated by rescue workers; no one was injured. The track in question has not been used since the derailment.

Goodine said his investigation has raised questions about Metro's maintenance, its training of track workers as well as train operators and controllers, and quality control of track work. He said it also has revealed problems at the transit agency with record-keeping as it applies to track use and history.

Goodine is expected to release a list of recommendations for improvements and a final report within the next week.