AFTER A quarter-century of planning, several hundred million dollars in public money, scores of public hearings and endless studies, the Purple Line, one of the Washington area’s most important transit projects, may be facing extinction. If that happens, it would be a testament to dysfunction, inertia and judicial negligence.

Having come within five days of receiving $900 million in federal funding, the 16-mile light-rail line was dealt an unwarranted setback last summer by a federal judge, whose ongoing foot-dragging, combined with the Trump administration’s hostility to new transit ventures, imperils an east-west link that would be a lifeline for tens of thousands of residents of Montgomery and Prince George’s counties and would revitalize an arc of close-in suburban communities.

With every passing day, the Purple Line’s prospects are dimming. The federal funding agreement frozen in August by U.S. District Judge Richard J. Leon was the project’s linchpin; without it, a multibillion-dollar public-private partnership cannot go forward, and investors who were ready to start building are stuck.

Without a green light now from Mr. Leon, it may be all but impossible to revive the federal funding agreement for the foreseeable future. That’s because the Trump administration has proposed halting all cash for transit projects that lack signed funding agreements, starting almost immediately and lasting for the remainder of the fiscal year.

Beyond that, there is no sign that the Trump administration is interested in improving the United States’ transit networks. To the contrary, the administration’s current stance suggests indifference toward transportation projects that don’t serve automobiles.

Mr. Leon halted the Purple Line’s federal funding on the grounds that the project had not adequately taken into account falling ridership levels on Metro, on which the Purple Line would depend for about a quarter of its passengers. But even if Metro’s ridership stagnates or continues to slip for another decade, which is far from certain, the net effect on Purple Line ridership would be modest. According to federal officials, the Purple Line would be among the busiest new transit projects in the country for which federal funding has been approved recently — regardless of Metro’s ridership levels. Mr. Leon’s ruling, which calls for a supplemental study of the Purple Line’s environmental impact, is at odds with Supreme Court precedent that cautions against ordering burdensome new studies in the absence of new information that paints “a seriously different picture of the environmental landscape.”

Fundamentally, the Purple Line makes sense as a transportation link for tens of thousands of daily riders who head to Bethesda, Silver Spring and College Park every workday. It makes sense as an economic catalyst for the region, which is why it has attracted significant private funding. Without the federal contribution, the Purple Line is dead, and so is a cornerstone of rational urban planning in the national capital region.