A federal appeals court ruled Tuesday that declining ridership on the Washington area's beleaguered subway system does not require Maryland to update the Purple Line's environmental study, allowing construction on the 16-mile light-rail line to continue.

The ruling by a three-judge panel for the U.S. Court of Appeals for the D.C. Circuit effectively ends a three-year legal battle over the project, unless its opponents try to appeal to the U.S. Supreme Court.

In an opinion written by Judge Judith W. Rogers, the panel said Purple Line opponents did not reveal any legal flaws in the project's federally required environmental analysis. Specifically, the court said opponents had not proven their argument that Metro's decline would affect the Purple Line's ridership projections and potentially the state's decision to build a light-rail line over a less expensive rapid-bus option.

Construction on the light-rail line started in August following a year of legal delays, after a different three-judge appeals panel allowed building to proceed while the appeal was pending. The line is scheduled to open in 2022.

The ruling reversed a 2016 order by U.S. District Judge Richard J. Leon that had required Maryland officials to hold off on building the Purple Line until they updated its ridership projections. Leon had agreed with Purple Line opponents that the state hadn't sufficiently considered Metro's safety and ridership woes.

However, Rogers wrote that Metro's ridership decline would not affect the project's environmental footprint. Moreover, she wrote, it was not significant "new information" that would require the Maryland Transit Administration to update the study and override the Federal Transit Administration's determination that doing so was unnecessary.

Rogers wrote, "Even if Metrorail ceased to exist — an extreme and highly unlikely scenario given its centrality to transportation in the greater Washington metropolitan area — light-rail would still provide faster (and higher-capacity) east-west connections between major Maryland activity centers in Montgomery and Prince George's counties than would other alternatives, like bus rapid transit."

The Purple Line, which will run inside the Capital Beltway between Bethesda in Montgomery County and New Carrollton in Prince George's County, will be operated separately from the Metro system. However, 27 percent of the Purple Line's riders are expected to be people transferring to or from Metro.

After Metro's ridership peaked in 2008 with 750,000 average weekday boardings, that number has fallen to just over 600,000, according to figures released in October.

Rogers added that, even with Metro's declining ridership, the Purple Line will still meet the state's goals to improve east-west public transit, better connect communities and spur economic development around stations.

Amelia Chasse, a spokeswoman for Maryland Gov. Larry Hogan (R), called the ruling a "win" for state and local governments.

"Once again, common sense has prevailed over the frivolous legal maneuverings of a handful of obstructionists looking to halt progress in our region," Chasse said.

The lawsuit, while not unusual for a major transportation project, had attracted national attention in part because the Purple Line is being built with a $5.6 billion public-private partnership, one of the largest such deals in the country.

The American Road & Transportation Builders Association had filed a brief in support of the state's appeal, saying that a ruling against the Purple Line would pose a "grave risk" to other U.S. transportation projects and be "an abuse" of the National Environmental Policy Act (NEPA).

"The ruling provides a clear notice that NEPA was not intended to be used as a tool to endlessly delay critically needed transportation infrastructure projects," the group said.

John Fitzgerald, an environmental lawyer and one of the lawsuit's plaintiffs, called the ruling "unfortunate." He said it's "quite unlikely" he and the other two plaintiffs — Chevy Chase resident Christine Real de Azua and the Friends of the Capital Crescent Trail — will appeal to the U.S. Supreme Court. Doing so, he said, would be "tough" and expensive for two citizens and a nonprofit group.

He said they will now focus on fighting their second lawsuit pending in U.S. District Court. That complaint alleges other flaws in the Purple Line project, including allegations that the federal government wasn't allowed to commit $900 million toward its construction when Washington-area governments are still struggling to pay for Metro's upkeep.

"There's a lot there to be done," Fitzgerald said.