A view of the Silver Spring Transit Center when it was under construction. (Yue Wu/The Washington Post)

For more than three decades, Parsons Brinckerhoff has been one of Metro’s go-to contractors, overseeing rehabilitation of tunnels and tracks and the design of stations and parking garages. Over the past five years, the transit agency has paid the global engineering giant $81.3 million to manage its capital improvements program.

But the company now has a new and less profitable relationship with the transit agency: defendant in a lawsuit.

Metro joined Montgomery County last month in suing its longtime partner for negligence, alleging poor design and management in construction of the Silver Spring Transit Center. The three-level bus, taxi and bike hub, adjacent to the Silver Spring Metro station, is scheduled to open early Sunday after five years of delays and $50 million in cost overruns.

Metro and Montgomery allege, among other issues, that the firm’s design was overly complex and rigid, leaving no allowance for concrete slabs to move as heavy vehicles roll by.

Metro contends that the deficiencies left long-term problems, including excessive cracking, that will make the facility more expensive to repair and maintain. It has asked for $25.8 million in damages. Montgomery and Metro also are suing the project’s general contractor and inspection firm for a total of more than $160 million. Metro, however, did not join the county in perhaps the most serious allegation against Parsons Brinckerhoff: that it misrepresented the work it had done to calculate potential pressures on the building’s interior beams and girders.

Parsons Brinckerhoff maintains that the design is sound and has been used in other projects, including parking garages at Cincinnati/Northern Kentucky International Airport and Boone Hospital Center in Columbia, Mo.

For a firm that designs rail lines in California and India and is part of the consortium that is rebuilding New York’s LaGuardia Airport, the transit center is strictly small potatoes. A defeat in the Maryland courts is unlikely to affect the company’s bottom line.

And a finding of negligence in the transit center matter would be far from its biggest black eye. In 2008, the firm joined the construction and engineering giant Bechtel in acknowledging oversight lapses in Boston’s Central Artery/Tunnel Project, also known as “The Big Dig.” The companies agreed to pay more than $400 million to the state of Massachusetts and the federal government for leaky tunnels and weaknesses that resulted in the death of a motorist when concrete and other material fell from a ceiling.

But a loss in the transit center case still would be a blemish for a company that has had a hand in some of the Washington region’s most consequential public works projects, including the Woodrow Wilson Bridge, the Intercounty Connector, the ongoing transformation of Tysons Corner and the proposed Purple Line light-rail project.

How the transit center lawsuit might affect Metro’s future dealings with Parsons Brinckerhoff is not clear. The capital improvements contract expires in May. Metro interim General Manager Jack Requa, citing the pending court action, declined to comment.

Cliff Eby, president of Parsons Brinckerhoff’s U.S. transportation company, said in a statement that he hopes the working relationship will continue despite the legal dispute.

“Parsons Brinckerhoff takes great pride in its long partnership with [the Washington Metropolitan Area Transit Authority] and the opportunity to participate in the agency’s accomplishments in the region,” Eby said. “We look forward to continuing that relationship for the benefit of WMATA and the region.”

There has already been one adjustment in the Metro-Parsons Brinckerhoff relationship, the severing of an arrangement that some critics on the Metro board said created at a minimum the appearance of impropriety.

About three months ago, Metro board Chairman Mortimer L. Downey quietly ended a 10-year relationship with Parsons Brinckerhoff as a “senior adviser” on mass transit matters.

Downey, a former deputy secretary of transportation during the Clinton administration and executive director of New York’s Metropolitan Transportation Authority, was a company consultant when he joined the Metro board in 2010. He succeeded Tom Downs as chairman in January.

Downey said in an interview that his duties included offering strategic advice on company projects, such as high-speed rail ventures in California and London. The contract initially called for eight days of work per month but was later cut back to four. He said the position paid about $100,000 a year.

But Downey, Metro and Parsons Brinckerhoff officials said there were bright lines separating his board duties from any corporate work.

“During his tenure, Parsons Brinckerhoff and Mr. Downey agreed that he would not assist us on WMATA projects,” Eby said. “We had a firm firewall that everyone respected and diligently maintained.”

Downey said in an interview that the relationship was reviewed annually by the agency’s general counsel. But with Metro under increasing criticism for serious lapses in its management, safety and fiscal practices, Downey severed the relationship with Parsons Brinckerhoff.

“The appearance issue was raised again, and everybody concluded that the best way . . . was to end the contractual relationship,” Downey said.

Board member Michael Goldman said Downey’s move was the proper one.

“I think it was the prudent thing to do, in light of the kind of scrutiny WMATA has been under,” Goldman said. “WMATA has enough problems.”

Joshua Schank, a former Parsons Brinckerhoff management consultant and president and chief executive of the Eno Center for Transportation, a nonpartisan think tank, said that while the transit center lawsuit will create tensions in the short run, Metro and the company are likely to do business in the future.

As a public entity, he said, Metro is obligated to accept all bids for work “without prejudice.”

“No one’s pleased about it,” Schank said. But over time, he added, key players on both sides will move to other jobs, to be replaced by others less invested in the matter.

The court battles could last nearly as long as construction, which began in 2008.

The general contractor on the job, Foulger-Pratt, which is also a defendant in the Metro-Montgomery lawsuit, is expected to file a counterclaim against the county and Parsons Brinckerhoff, alleging that poor management and shoddy design — not its work — produced costly delays.

The lawsuit is assigned to Montgomery County Circuit Court’s business and technology track, which allows an extended schedule for unusually complex cases. It provides for more than 400 days between the filing of a case and the beginning of trial, much of which both sides are expected to spend in discovery. The county doesn’t expect a trial to begin until early 2017, and estimates it could last a year to 18 months. Appeals could take two years.

Montgomery requires parties in the suit to enter “alternative dispute resolution,” or mediation, a process that could lead to an out-of-court settlement. But such a settlement makes it likely that Montgomery County would fall short of the 2013 reelection campaign promise by County Executive Isiah Leggett (D): that every taxpayer dollar spent fixing the transit center’s defects would be recovered.

“It’ll be a challenge,” said Matthew J. Pavlides, an attorney for the firm Miles and Stockbridge who specializes in construction litigation.