Artist rendering of the interior of planned Purple Line commuter trains in Maryland. (Purple Line Transit Partners)

ONE OF the Washington area’s most important new transit projects is now imperiled by an unfounded ruling by a federal judge. With a tossed-off opinion that ignores the plain language of federal law and Supreme Court precedent, U.S. District Judge Richard J. Leon derailed for the time being, and maybe indefinitely, nearly $1 billion of financing for the Purple Line, the 16-mile light rail project that would provide an east-west link for residents of Montgomery and Prince George’s counties.

In a nine-page opinion Wednesday, the judge cast aside the judgment of federal and state officials who have pored over the Purple Line’s details for years, irked that they have failed to take into account the safety travails and slipping ridership of Metro, four of whose branches would connect to the Purple Line.

Although Metro would not build, own or operate the project, it would provide just over a quarter of Purple Line passengers. Since Metro’s ridership forecasts are now in doubt, the judge said, so are ridership and revenue estimates on which federal and state approvals for the Purple Line are based.

Judge Leon’s ruling rests on what he sees as the need for an additional environmental impact study, to supplement one done several years ago that relied on outdated ridership projections. However, at no point does he explain how slipping Metro ridership would be relevant to the Purple Line’s environmental impact.

That’s a major omission given that the Supreme Court specifically cautions against ordering new environmental studies except when “new information provides a seriously different picture of the environmental landscape.” Yet by ordering a new environmental study, the judge forced federal officials to cancel a ceremony Monday finalizing $900 million in federal aid, nearly half the project’s $2 billion construction cost.

It’s hard to see how Metro’s problems, now the focus of mounting federal intervention, would pose a major long-term threat to the Purple Line, which had been expected to begin operations in 2022. Even if Metro ridership falls short of projections by 10 percent for the next five or 10 years — a big if — that means Purple Line ridership might slip by slightly more than 2.5 percent. Not exactly a lethal blow.

Granted, Maryland transit officials have not been sufficiently transparent about how they arrived at ridership numbers. And if their estimates for the Purple Line fall short of forecasts, it could prompt the state to raid other revenue sources to pay off the project’s construction debt.

But every transit project runs the risk of revenue shortfalls. No evidence has surfaced that Metro’s problems pose such a threat to the Purple Line’s long-term prospects as to warrant its cancellation or justify another approach entirely (such as an express bus line). Nor is there any compelling case that federal and state officials acted irresponsibly in declining to launch a new, months-long environmental review based on finger-in-the-wind speculation about passenger forecasts decades into the future.

Opponents of the Purple Line, led by well-heeled locals who fear its impact on a private golf course and a public trail, have thrown every possible obstacle in the project’s path. Now one has stuck, to the detriment of the region’s economic prospects and commuters.