Clarification: An earlier version of this editorial stated that the private consortium contracted to build and then operate the Purple Line would “chip in $1 billion” to help pay for construction. The companies are financing $1 billion of the line’s construction costs; Maryland will repay the private debt service as part of the pre-set annual payments that will average $150 million a year after the line opens. This version has been updated.

IN RECENT years, as Maryland’s proposed light-rail Purple Line drew closer to final approval after decades of planning, it faced a rolling battery of existential threats. They included the initial hostility of a Republican governor; the die-hard opposition of well-heeled suburban neighbors who feared losing portions of a beloved walking and biking trail; the Trump administration’s threat to ax transit funding nationally; and an exceptionally obtuse ruling by a federal judge that for a time seemed the death knell for the embattled $5.6 billion project.

Despite those hurdles, the Purple Line remains very much alive — so alive that on Monday it celebrated a groundbreaking attended by Gov. Larry Hogan (R) and federal Transportation Secretary Elaine Chao, who also signed a $900 million federal funding agreement. Mr. Hogan, who as a candidate pronounced the project unaffordable, now says construction will start immediately. To leave no doubt, he promptly took controls of an excavator and started whacking away at a building to be razed to make way for the project.

The Purple Line is not yet in the clear. A federal appeals court has not finished hearing from the project’s opponents, mainly some of its neighbors in Montgomery County who still hope to halt the 16.2-mile line that would link Bethesda in Montgomery County with New Carrollton in Prince George’s County. Nonetheless, the court, which swept aside a lower-court ruling blocking federal funding and stopping construction, gave the go-ahead for both. Optimism now surrounds the Purple Line’s prospects and its promise of improving the lives of tens of thousands of daily commuters. Older, close-in suburban communities through which the line’s 21 stations would be built also have reason to hope for economic improvements.

In a public landscape littered with toxic partisan warfare and government dysfunction, the Purple Line’s construction would be a striking success story, a rare example of broad cooperation across party lines, and among federal, state and local government partners in coordination with the private sector. In addition to the federal funding contribution, Maryland would kick in several billion dollars over 35 or so years; the two counties would also pony up tens of millions. A private consortium that would build and operate the project would chip in $1 billion, with expectations of long-term profits and Maryland repaying the private debt service as part of pre-set annual payments.

Everyone involved has had to swallow hard, not least Mr. Hogan, a Republican with a rural and exurban base who needed to cooperate with the two most heavily Democratic and urbanized counties in the state. The Trump administration has had to overlook Mr. Hogan’s plain distaste for the occupant of the Oval Office. And, yes, some (though not all) neighbors of the Purple Line have also realized that its obvious benefits outweigh the painful fact that a segment of it will run alongside a reshaped Capital Crescent Trail, diminishing a cherished local amenity in favor of a greater regional good.

Sometimes things in the public sphere actually work out for the best. Here’s hoping this is one of those times.