A U.S. District judge reaffirmed Tuesday that construction cannot begin on Maryland’s Purple Line until federal officials determine the impact Metro’s declining ridership could have on the light-rail line that will connect to the subway.

Judge Richard J. Leon, however, backed off on part of his initial ruling in August, when he ordered the Federal Transit Administration to also reopen the project’s entire federally approved environmental review when considering Metro’s potential impact. Updating that larger study could add months of further delay to the Purple Line project, which saw its October construction start put on hold after the August ruling.

Leon said he would reverse himself on that point because he agreed with government lawyers who argued that federal transit officials, not a judge, have the expertise necessary to decide whether the estimated impact of Metro’s declining ridership warranted reopening the larger environmental study.

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In reviewing his earlier decision, Leon said he once again noted the federal and state transit agencies’ “seemingly cavalier attitude” in dismissing any potential ridership impact by saying that the Purple Line will be operated separately from Metro.

“The seriousness of this deficiency is underscored by the size, scope and cost of this major infrastructure project,” the judge wrote, referring to Maryland officials seeking $900 million in federal aid to cover nearly half of the 16-mile project’s $2 billion construction cost.

The Purple Line will be owned by the Maryland Transit Administration (MTA) and operated as a slower light-rail line, separate from Metro’s heavy-rail system. But Maryland officials have long touted the Purple Line as providing a vital east-west link between the state’s four spokes of the Metro system in Montgomery and Prince George’s counties. About 27 percent of the Purple Line’s ridership — the line is now scheduled to begin carrying passengers in 2022 — is projected to come from people transferring to and from Metro.

Metro ridership has fallen 12 percent since 2010, with 100,000 fewer trips per day.

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The MTA estimated the Purple Line’s long-term ridership forecasts as part of a years-long environmental review to determine the project’s costs and benefits before it decided to build a light-rail line over a less expensive bus option.

MTA spokesman Ryan Nawrocki said the agency needed more time to review the late-afternoon ruling before commenting. A spokesman for the Justice Department, which represents the FTA, did not return an email seeking comment.

Ajay Bhatt, president of the Friends of the Capital Crescent Trail advocacy group, one of three plaintiffs in the case, said he had not had a chance to read Leon’s latest decision. But he said he is eager to see the numbers that FTA cites to back up its previous statements that Metro’s declining ridership and safety problems would not have a significant effect on the Purple Line’s long-term ridership.

“We want to see how they added one plus one and got five,” Bhatt said.

The judge said the FTA should assess Metro’s potential impacts “as expeditiously as possible” and report back to him on whether they warrant an updated environmental review. If the FTA determines that redoing the larger study isn’t necessary, both sides can further argue whether the lawsuit should be dismissed based on the Metro ridership issue.