Richard Sarles talks to reporters at a Metro facility in Bladensburg, Md., in this file photo. (James A. Parcell for The Washington Post)

Richard Sarles, whose tenure as head of the nation’s second-busiest subway system began in the aftermath of the fatal 2009 Red Line crash that killed nine people, announced Wednesday that he is stepping down as chief executive.

Sarles stunned many of Metro’s board members with his decision, saying simply that it was time to go.

“I’m about to hit 70,” he said. “Time is getting precious to me. I really want to enjoy my family. I want to do some things that I really haven’t had the time or opportunity to do.”

Sarles will leave a system that is in far better shape when he took over nearly five years ago.

In July, officials celebrated the opening of the first phase of the $5.6 billion Silver Line rail extension, its first line in more than two decades. And this week, officials announced a return to automatic train operation on selected portions of the Red Line — a sign many say demonstrates the authority has put the worst behind it.

Commuters make their way through the Wiehle-Reston East station during the first business day of operation for the Silver Line on July 28 in Reston. (Amanda Voisard/For the Washington Post)

That may be why Sarles’s announcement that he will be stepping down in January left so many surprised.

Mortimer L. Downey, a former deputy U.S. transportation secretary who represents the federal government on the board, said he and other members were “kind of shocked and stunned” by Sarles’s decision.

Last year, the board voted to extend his contract through January 2016. His annual salary rose from $350,000 to $366,000. At the time, Sarles said he wanted to stay because he enjoyed the job, and while Metro was improving, there was still more work to be done.

But Sarles said he changed his mind following an August vacation to the Jersey Shore. He said he wanted to rebalance his life and spend more time with his eight grandchildren.

He said he hoped his successor would continue lobbying for Washington-area jurisdictions to provide “full funding for the Metro 2025 program, so we can have all eight-car trains” in the subway system.

“That’s what this region needs over the next several years, and we have to deliver that,” he added.

Metro board chairman Tom Downs praised Sarles’s tenure, saying he led the agency out of one of its darkest chapters, the 2009 Red Line crash in which nine people were killed and dozens were injured.

Sarles, an engineer by training, was a low-key leader who eschewed the spotlight. He took the helm of the agency when it was under fire for what federal officials blasted as a lax safety culture.

“He was a buttoned-down workhorse who restored confidence in the system after a very rocky run,” said U.S. Rep. Gerald E. Connolly (D-Va.).

Sarles also oversaw the massive rebuilding of the 38-year-old system. For riders, that has meant the all-too-frequent closing of stations and long waits for trains and shuttle buses on weekends. That work also has coincided with recent declines in rail ridership.

In 2013, Sarles — with an eye on Metro’s future — announced an ambitious campaign, dubbed Momentum, to raise $26 billion to rebuild and expand service.

“He helped advance the region’s transportation priorities and supported the primary focus on safety, bringing the Metro system back to a state of good repair, and a well-conceived vision for Metro’s future,’’ said Chuck Bean, executive director of the Metropolitan Washington Council of Governments.

But it was the July opening of the Silver Line that has given the biggest boost to Metro’s plans for its future. On Wednesday, Metro officials released statistics showing that it is already more than halfway to its goal of 25,000 daily boardings.

Sen. Barbara A. Mikulski (D-Md.), a Metro booster who also can be one of its toughest critics, on Wednesday thanked Sarles for his service.

“He became CEO and General Manager during Metro’s darkest days,” she said in an e-mailed statement. “But with unflagging energy, he implemented the National Transportation Safety Board and Federal Transit Administration’s safety recommendations. He will be missed and I wish him well in the next chapter of his life.”

Even as he is credited with working aggressively to rebuild the system and address safety issues, Sarles’s departure comes amid troubling reports of mismanagement on Metro’s contracting and procurement side.

In August, Metro officials were forced to pay nearly $5 million to settle a whistleblower lawsuit after a former employee alleged that the transit agency violated federal rules by awarding a $14 million contract without seeking competitive bids.

Earlier this year, a federal oversight report found that Metro might have mishandled billions in federal grant money, violated rules for contracting and procurement by awarding contracts without seeking multiple bids — and in some cases breaking them into smaller amounts — in what appeared to be an effort to skirt reporting requirements. As a result, the Federal Transit Administration restricted Metro’s ability to draw on federal funds, forcing the authority to borrow money to cover its bills.

Metro’s chief financial officer Carol Kissal, who was named in the whistleblower suit and oversaw contracts and procurement during the period examined by federal officials, later left the authority, although Sarles said her departure had nothing to do with the critical audit.

When Sarles was appointed interim chief executive, then-Metro board chairman Peter Benjamin told The Washington Post that board members recruited him because he had two key characteristics — he was someone who knew transit well and he could step in and be more than a caretaker. The board, Benjamin said, wanted “ an assertive and aggressive leader of the organization because we have a lot of issues we have to tackle, starting with safety.”

Sarles replaced John B. Catoe, who resigned in the wake of the 2009 Red Line crash.

Downs said the board plans to conduct a nationwide search for Sarles’s replacement.

Prior to coming to Metro, Sarles served as head of New Jersey Transit. He retired from the position in January 2010 and was hired as interim chief executive at Metro in March of that year. He was made Metro’s permanent CEO the following year.

Fairfax Board of Supervisors Chairman Sharon Bulova (D) said Sarles leaves Metro in better shape than when he arrived.

“There was a culture in the agency among the workers and management that was not attuned to safety and customer service,” she said. “He’s really stepped up to the plate to change those things. Has everybody arrived? Is everything perfect? No. But there’s been a change for the positive.”