In the three years since Metrorail’s deadliest crash, the transit authority that operates the system has doubled its staff of safety experts, improved its train-detection systems and begun the process of replacing some of the oldest cars in its fleet.

Still, the authority has continued to struggle with safety problems.

In recent months, a Blue Line train derailed in Rosslyn; a Metro mechanic was seriously injured when struck by a train in a maintenance yard; and the doors of a moving Red Line train opened without warning. A passenger trying to help a heart attack victim in April found that a station’s defibrillator didn’t work. Internal reports unearthed by the media this month show the agency ignored inspectors’ repeated warnings about inaccessible emergency exits. Rails have cracked with increasing frequency.

On Wednesday, for the third time since December, Metro reported that a part of the brake system had fallen off a rail car.

Overall, Metro watchdog groups and public officials say, Metro is safer than it was on June 22, 2009, when a faulty automated-signal system failed to detect a Red Line train and sent another train crashing into it near Fort Totten. Nine people died, and dozens of others were injured. However, safety monitors say there’s room for improvement.

Deborah A.P. Hersman, chairman of the National Transportation Safety Board, said Metro has been more receptive to the board’s recommendations.

“Before the Fort Totten accident, the NTSB had been very frustrated because we’d investigate accidents on Metro and we wouldn’t see a response to our recommendations or attention to safety,” Hersman said. “But since the Fort Totten accident, we’ve seen Metro really embrace our recommendations. . . . I think we’ve seen a real change in attitude with respect to safety.”

Matthew Bassett, outgoing chairman of the Tri-State Oversight Committee, an independent watchdog agency, said Metro has made “fairly strong progress” on safety but has more to do.

Of particular concern, some transit experts say, is that the problems have occurred even though Metro has received hundreds of millions of dollars in additional government funds for safety improvements and rehabilitation projects. Metrorail, the nation’s second-busiest subway system, will also face additional pressures when the first, 11.5-mile section of the Silver Line opens to Tysons Corner late next year.

“I do think these smaller events are a sign that they need to pay attention and stay on top of these things,” Hersman said.

Watchdog groups and public officials say the Washington Metropolitan Area Transit Authority, which operates Metro, deserves credit for its faster response to safety problems in recent years. And improvements are being made as Metro, like other large U.S. rail systems, struggles with aging equipment and decaying infrastructure, transit experts say.

Metro has more than doubled its safety office staff, from 28 to 60 people, and it has embarked on an ambitious training program for workers and reduced injury rates for passengers and workers, agency officials say. Customers say they’ve noticed some improvements, from replaced broken station tiles to fewer escalator breakdowns.

General Manager Richard Sarles, who runs Metro, was not available to speak about the changes at Metro since the crash.

But the authority’s spokesman offered this assessment.

“Today Metro is a safer system than it was in 2009 but it will take more investment and more hard work in the coming years to get us to where we want to be,” Dan Stessel said.

Three years ago, Metro did not have a separate department to deal with safety issues. Today, a new chief safety officer, who reports directly to Sarles, heads a team of 60 staff members. Their charge is to spend most of their time in the field. The Metro board’s safety and security committee now receives detailed briefings on all incidents.

“The safety department was pretty dismal,” said Mort Downey, who chairs the committee. “That’s all changed.”

Officials have addressed eight of the 15 NTSB recommendations related to the 2009 crash. But others, including the replacement of the 1000 series rail cars that the NTSB has criticized for offering too little protection in crashes, will take more time to resolve, Stessel said. He added that the agency is spending $765 million on 364 new rail cars — 300 that will replace the 1000-series cars and 64 for phase one of the Silver Line. The first cars are expected to arrive in late 2013.

Metro has improved its train-detection systems; it is in the process of replacing more than 1,500 track-circuit modules, with more work to be done. But trains will continue to operate in manual mode until more testing is completed, Stessel said. “We can’t yet say we’re totally comfortable with the control system, its integrity and its fail-safe characteristics,” Downey added.

Jackie Jeter, president of Amalgamated Transit Union Local 689, said the authority has invested heavily in safety training but more can be done to improve the culture at Metro, including holding supervisors to the same standards as front-line workers.

“You don’t fix anything of this magnitude overnight,” said Jeter, whose union represents most Metro workers. “They are on the road to recovery, but they’re not quite there.”

In the meantime, legislation prompted by the crash that would establish federal safety standards for subway and light-rail systems nationwide is linked to congressional negotiations over a broader transportation funding bill.

U.S. Sen. Barbara A. Mikulski (D-Md.) said she thinks Metro officials have become more vigilant about safety, but she said she was “very distressed” to hear that doors on two Red Line cars opened without warning during a morning commute in May.

“My mouth fell open in shock,” Mikulski said. “I was terrified for the riders, but I think we are making progress in replacing those cars.”

Stessel said the door-opening was an isolated incident caused by a damaged coupler, a mechanical device that connects one car to another. Officials inspected the rest of the fleet and found no other problems, he said.

Some passengers at the Fort Totten station said they feel relatively safe when riding the Red Line.

“To me, Metro gets a lot of bashing,” said Kelly White, 53, a publication coordinator at the American Public Health Association, during his morning commute to Gallery Place. But White said he has avoided sitting in the front or back of Metro trains since the crash.

“I’m conscious of that, for sure,” White said.

Many families — still mourning loved ones — will gather Friday for a memorial service that includes the dedication of a plaque in memory of crash victims. Carolyn Jenkins, whose daughter Veronica DuBose, died in the crash, plans to be there.

“It’s been a hard road,” she said.

Transportation experts say it will take years to overhaul the culture of a large agency that must shift from building a 106-mile system to operating and maintaining an aging one. Hersman, of the NTSB, said Metro’s attention to safety requires constant vigilance, the kind that the anniversary of the crash should inspire.

“Certainly, it’s a painful time for the families, and it’s a time of reflection for the community,” Hersman said. “I think for Metro, this is a time for them to reevaluate where they are and where they need to go.”

Staff writer Mihir Zaveri and staff researcher Magda Jean-Louis contributed to this report.