A contractor walks to cut down a tree and clear out space along the Georgetown Branch Trail to make way for the Purple Line on Sept. 27, 2017 in Chevy Chase, Md. (Ricky Carioti/The Washington Post)

Federal judges deciding if Maryland may continue building the Purple Line focused Wednesday on a key question: whether the drop in ridership on Washington's troubled subway should force the state to reconsider how many people will ride its light-rail line.

The state justified building the $2.4-billion light-rail line to improve east-west transit in the densely developed D.C. suburbs in part by saying it would carry more passengers long-term than bus options. Purple Line opponents say the state must update those ridership numbers and reconsider buses, which they say would be cheaper and less environmentally damaging.

If the U.S. Court of Appeals for the D.C. Circuit agrees with Purple Line opponents, it could require the state to stop some construction work, such as tree-cutting that began this fall, until the project's environmental impact study is updated.

A three-judge panel is expected to rule in coming weeks.

In oral arguments Wednesday, the judges pressed a lawyer for the Maryland Transit Administration about the state's rationale for building the line: to provide faster east-west transit, link communities around the 21 stations, and better connect people to Metro. A light-rail line between Montgomery and Prince George's counties will meet those three needs even if Metro ceased to exist, a lawyer for the state said.

What if, due to Metro's falling ridership, one "leg" of the three-legged stool collapsed? the judges asked.

"One of the purposes falls out — what happens?" Judge Judith W. Rogers said.

Rogers asked whether ridership forecasts should be updated by law or become a question for government officials to decide.

"We're pushing to understand where the [federal law] line ends and the wisdom of a project begins," Rogers said.

The judges appeared well aware of Metro's problems, even if no one brought up the numbers. After peaking in 2008 with 750,000 average weekday boardings, ridership is just over 600,000 average weekday boardings, according to figures released last month.

Metro officials have attributed the trend to commuters teleworking more and people avoiding the aging subway's chronic maintenance and reliability problems.

Maryland is appealing a lower-court ruling in favor of Purple Line opponents' arguments that the state didn't sufficiently consider Metro's decline when it estimated the Purple Line's ridership.

That ruling delayed the start of light-rail construction by a year.

However, a different three-judge appeals panel allowed construction to begin this summer while the appeal is heard. The line is scheduled to begin carrying passengers in 2022.

The plaintiffs in the 2014 lawsuit — the Friends of the Capital Crescent Trail and two Chevy Chase residents, John M. Fitzgerald and Christine Real de Azua — are appealing, too.

They say the lower-court judge erred in not finding other flaws in the environmental review.

For example, they said, the state hadn't considered the potential environmental impacts of installing a gravel track bed rather than "green tracks" previously intended to control storm water runoff into nearby streams.

Getting rid of the green track "warrants some additional scrutiny," said Eric R. Glitzenstein, a lawyer for the opponents.

But it was Metro that captured the judges' attention.

The 16-mile Purple Line will operate separately from Metro, but Maryland officials say 27 percent of its riders will be Metro passengers.

Chief Judge Merrick B. Garland asked a lawyer for the Federal Transit Administration whether a "dramatic change" in ridership could affect the environmental impacts of two transit options under consideration.

"Is that the kind of change in impacts the law requires you to analyze?" Garland asked.

The judge continued, "You studied [improved bus service] under an assumption of a number of riders that is no longer good."

Albert M. Ferlo, a private lawyer for the Maryland Transit Administration, said the state had already analyzed Metro's decline and determined that a light-rail line was still warranted, even if it carried no Metro passengers.

Even so, Ferlo said, federal law required agencies to consider only "circumstances that are reasonably foreseeable" — and it's not reasonable to assume Metro will disappear.

"Yes, there are acknowledged problems [with] Metro, but those are problems that the three jurisdictions are working together — or at least trying to work together — to solve," Ferlo told the panel.

The judges also had tough questions for Purple Line opponents. Rogers said it appeared the state already had done a "thorough analysis" of the Purple Line's potential impacts.

She said it seemed Purple Line opponents were asking the judges "to reconsider some of the wisdom" of the state's decision to build a light-rail line rather than providing "new information" that required further study.

She noted the state had already considered Metro's decline, even if opponents' ridership experts disagreed about its impact.

"What's new?" Rogers asked.

"It's not been adequately addressed," Glitzenstein, the opponents' lawyer, said.