Metro was "not sufficiently prepared" to prevent massive problems this year that caused a train to derail, stranded riders and prompted some to evacuate a train on their own, according to a federal safety audit released Wednesday.

The Federal Transit Administration, or FTA, looked at nine Metro safety incidents that occurred since last December, ranging from a derailment to a communication blackout to doors opening while trains were running. It was a follow-up to a damning audit that federal investigators conducted in 2010 in the wake of multiple deadly accidents, including the 2009 Fort Totten train crash.

This time, auditors said they were concerned that recent track work, including the way rail is installed with continuous welding, may have contributed to the July 6 derailment that was caused when the rail warped during a summer heat wave. They also found hazards in rail yards and shops, such as exposed electrified equipment, open pits and rail car doors without fall protection after a worker was hit by a train in a rail car washing station.

Recent Metro incidents reviewed
December 2011-June 2012: Four cases of brake discs, known as friction rings, falling from Metro trains.
May: Doors opening on moving trains while not at the platform, including "confirmed and unconfirmed" cases.
May 29: Metro worker struck by a train in a car wash facility at the Shady Grove rail yard.
July 3: Self-evacuation of passengers from a stalled train near College Park stop on the Green Line.
July 6: Green Line train derailment attributed to a "heat kink" of the track.
July 14-15: Control center computer network outage.

Part of the problem has been a flood of employees retiring within the past 18 months, leaving new workers to handle Metro's complex systems, which take two to five years to master. "[W]ith so many new employees on the system, unusual occurrences and unexpected failures in system components were not always handled as well as they should have been, particularly when communicating with passengers and the public," the audit said.

Most of the track-related departments are averaging vacancy rates from 10 to 12 percent, the audit said.

Even so, the investigators said Metro has made "considerable progress" in improving its safety oversight and its communications about problems. However, the auditors noted it's a "critical but fragile foundation for the future" for the Washington Metropolitan Area Transit Authority.

"Any major changes in personnel, relaxation in attention to safety, reduction of resources devoted to safety, or reemergence of complacency in the reporting and investigation of unusual occurrences and conditions could easily reverse WMATA's recent gains," the audit said.

The FTA made six recommendations to "address remaining gaps" highlighted by the incidents. Metro said Wednesday it has already begun taking steps to meet the recommendations.

"We welcome the recognition of our significant safety progress in this report andagree that vigilance is key to continuing our progress," said Jack Requa, who was serving as Metro's acting general manager while Richard Sarles was out of town.

U.S. Sen. Barbara Mikulski, who requested the audit this summer, praised Metro for starting to change its ways. "But I insist Metro continue making progress in creating a culture of safety," the Maryland Democrat said.

"There were computer failures causing all five Metro lines to shut down twice over a single weekend, train derailments, train doors opening while moving, blocked and locked emergency exits, brake pieces falling off trains, unacceptable operator communication leading to very dangerous passenger evacuations and Metro worker injuries," she said. "People on the trains were terrified. And people watching it unfold on the TV news were losing confidence in Metro."