An artist’s rendering of the interior of the planned Purple Line commuter trains in Maryland. (Purple Line Transit Partners)

Construction of the light-rail Purple Line in the Washington suburbs could be delayed based on a federal judge’s ruling Wednesday requiring that the project’s ridership forecasts be redone to account for Metro’s deterioration and declining ridership.

U.S. District Judge Richard J. Leon ordered that the Maryland Transit Administration’s “record of decision” on the Purple Line — the federal stamp of approval on the project’s environmental analysis — be set aside until the state recalculates the ridership projections done for that study.

Leon said he couldn’t “turn a blind eye to the recent extraordinary events involving seemingly endless Metrorail breakdowns and safety issues.”

The Purple Line will not be part of Metro, but it will connect to four Metro stations, and more than a quarter of the Purple Line’s riders are expected to be Metro passengers feeding into the light-rail system. The lawsuit before Leon was brought by Purple Line opponents who said state and federal agencies had given short shrift to the line’s potential environmental impacts.

Leon wrote that the Federal Transit Administration showed a “cavalier attitude” when it argued that Metro’s safety lapses and falling ridership — which he called “serious issues” — will not affect the Purple Line because it will be separately owned and operated. Metro’s problems, the judge said, pose “serious questions” about the Purple Line’s “future viability.”

The FTA’s dismissal of Metro’s potential impact on Purple Line ridership “raises troubling concerns about their competence as stewards of nearly a billion dollars of federal taxpayers’ funds,” he said.

Maryland is seeking $900 million in federal transit construction aid for the 16.2-mile line. Major construction had been scheduled to begin late this fall.

Whether Leon’s decision will delay that start is not clear. However, during a June 15 hearing in the case, attorneys for the MTA and the federal government said refiguring the ridership forecasts could push back the project by six months and unravel its $5.6 billion, 36-year public-
private partnership.

In a subsequent court filing, lawyers for the Maryland Attorney General’s Office said having to redo the ridership forecasts “would be profoundly disruptive and could jeopardize the Purple Line project” because it could “have cascading consequences on the project schedule and financing arrangements.”

Maryland Transportation Secretary Pete K. Rahn said the state will appeal the ruling.

“We are deeply disappointed that this puts the Purple Line in jeopardy,” Rahn said in a statement. “We will work closely with the Attorney General to seek a quick decision from the court of appeals.”

Maryland officials had scheduled a public event for Monday to sign a funding agreement in which federal officials are expected to commit $900 million in grants toward the project’s $2 billion construction, according to three officials with knowledge of the event. The signing of that agreement was considered the last major hurdle before construction could begin.

The Purple Line, with 21 stations between Bethesda in Montgomery County and New Carrollton in Prince George’s County, is to be built and operated by the MTA. However, passengers transferring to and from Metro were projected to make up 27 percent of Purple Line riders.

The ridership forecasts were done as part of the state’s cost-per-rider analysis before it decided in 2013 to build a light-rail line instead of a less-expensive bus option.

John M. Fitzgerald, a Chevy Chase lawyer and one of the plaintiffs, said he was “heartened” by the judge’s finding that proceeding with the Purple Line before considering Metro’s potential impacts “would make no sense.” Fitzgerald said he hopes state and federal officials also will reexamine cheaper options, such as a rapid bus system, and consider holding off on building a Purple Line until Metro has been rehabilitated.

Maryland officials have said they think Metro’s backlog of maintenance and safety work will be done by 2022, when the Purple Line is scheduled to open to passengers. By then, they have said, Metro passengers who have stopped riding because of safety concerns and un­reliable service will have returned. After peaking at 225 million passenger trips in 2009, Metro ridership began to fall and has continued to drop.

The Purple Line, Maryland officials also have said, will run on different technology than the 40-year-old, heavy-rail Metro system. Its smaller trains will travel more slowly than Metro, mostly along local streets and powered by an overhead electrical line.

However, Maryland officials also have long argued that the Purple Line is needed, in part, to provide a faster, more reliable connection to the state’s spokes of the Metro system.

Fitzgerald and the other plaintiffs — the Friends of the Capital Crescent Trail and Chevy Chase resident Christine Real de Azua — filed the lawsuit in 2014. They argued that Maryland’s environmental review of the project and the federal government’s approval of that study were flawed.

After Metro suffered high-profile safety and maintenance lapses, the plaintiffs recently increased their emphasis on their argument that the Purple Line ridership forecasts were flawed because they presumed a “robust” Metro system.

Spencer S. Hsu contributed to this report.