LAST FALL, the federal judge who blocked funding and construction of the light-rail Purple Line in Maryland’s suburbs ordered federal officials to go back to the drawing board. The judge, Richard J. Leon, wondered if safety and maintenance problems on Metro, which would account for about a quarter of the Purple Line’s passengers, required a rethink of the new project’s finances and forecasts.

The answer was delivered in December: No, said the Federal Transit Administration, Metro’s challenges, including falling ridership, would not materially change the Purple Line’s prospects or warrant a months-long delay to update its environmental study — which opponents sought as a delaying tactic. In fact, said the FTA, even if Metro didn’t exist, the Purple Line would still be “one of the most robust light-rail systems” the feds had helped fund in years. As for the environmental impact of daily ridership on the Purple Line that might slip by a few thousand riders, the FTA’s assessment was: zero.

That should have been enough to settle Mr. Leon’s doubts, but five months later, with the clock ticking on crucial state and federal financing, the judge is mum. And the Purple Line, which was shovel-ready and poised to receive $900 million in federal funds last summer before he intervened, is now facing death by judicial inertia.

Mr. Leon’s stance is reckless and irresponsible. By ignoring the project’s financing timetable, he has, as Maryland Attorney General Brian E. Frosh’s office wrote last week , brought the region’s most important transit project “to the brink of cancellation.”

The Democratic attorney general’s warning came as part of the state’s request that a federal appeals court require that Mr. Leon render his decision on a three-year-old lawsuit filed by Purple Line opponents. Without a ruling in the next two weeks, the state is likely to halt design and engineering work rather than continue to bear the burden of $13 million in added costs for each month of delay occasioned by the judge’s inaction, according to the attorney general’s filing.

Funding for the rail project — a $5.6 billion link that would connect Bethesda in Montgomery County with New Carrollton in Prince George’s County and provide a badly needed economic boost to scruffy, close-in suburban neighborhoods — is a delicate patchwork from state, federal and private sources. It is at risk because a single judge won’t do his job.

And on top of the questions raised about Metro’s impact on the Purple Line’s ridership projections, Mr. Leon has failed to rule on 23 additional issues. That’s right: 23.

The stately pace the judge has set for himself is divorced from the real world and from the needs of tens of thousands of real people whose daily lives would be enhanced by the Purple Line. When he froze the project last year, Mr. Leon blasted federal and state officials for what he called their “seemingly cavalier attitude” in ignoring the potential impact of Metro’s falling ridership on the light-rail project. In fact, it is the judge’s own attitude that is cavalier.