A Purple Line map (The Washington Post)

A FEDERAL appeals court on Wednesday appeared to resurrect what would be the Washington area’s most important new transit link since the Silver Line — the light-rail Purple Line, which will connect Maryland’s close-in suburbs — by lifting a stay imposed this spring by a fuzzy-headed federal judge. The appeals court’s order gives the state of Maryland a fighting chance at reclaiming $900 million in federal funding for the Purple Line that was blocked at the 11th hour by U.S. District Judge Richard J. Leon.

At last, judicial common sense may revive a critical project nearly derailed by Mr. Leon, who ruled this spring that state and federal officials hadn’t seriously considered the potential effect of declining ridership on Metro, from which the new light-rail line expects to draw many of its users.

The U.S. Court of Appeals for the District of Columbia Circuit reinstated the project’s environmental approval, which Mr. Leon had invalidated based on the risible proposition that transit officials are clairvoyants who had failed to employ their powers to foresee Metro’s ridership levels far into the future. In fact, no one foretold the steep passenger loss that Metro suffered starting in 2011; and, now that the subway system has invested heavily in track repair as well as marketing, no one can predict whether ridership will recover, and if so at what pace.

As it happens, just a few weeks after Mr. Leon’s ruling, Metrorail had its five highest weekday ridership days of the year, including one (June 27) when the passenger count jumped to nearly 700,000, more than 10 percent above average counts a year earlier and nearly as high as the average for that period two years ago.

What is known, and what the Federal Transit Administration has quite reasonably argued, is that even under pessimistic scenarios — in fact, even if zero passengers transfered from Metro — the Purple Line would still be among the most heavily used light-rail projects to win federal funding nationally in recent years.

The FTA joined Maryland in appealing Mr. Leon’s ruling, which flew in the face of the Supreme Court’s admonition that judges should not substitute their views for the expertise of federal officials in cases, like this one, involving environmental studies.

In the meantime, the FTA’s original approach — offering five scenarios for future Metro (and Purple Line) ridership, ranging from moderate growth to continued decline — continues to look eminently sensible. It is silly to look at a few days’ ridership numbers and take those as evidence of a trend. It was equally silly for Mr. Leon to examine a few years’ decline in ridership, following decades of underinvestment in Metro, and assume an irreversible pattern.

The lawsuit seeking to block the Purple Line will continue for now, even as the state seeks to unlock the federal funds. The appeals court must still issue a ruling on its underlying claims. But amid all the legal wrangling, there is no doubt of the project’s fundamental appeal: It would provide an enormous convenience to tens of thousands of daily commuters and a means of revitalizing aging neighborhoods in its path.