A federal appeals court ruled Thursday that construction of Maryland’s light-rail Purple Line does not violate Clean Water Act protections for streams and wetlands.

The unanimous ruling in the third, and final, lawsuit by opponents of the 16-mile project in the Washington suburbs lifts the threat of legal action against it for the first time since 2014.

The U.S. Court of Appeals for the 4th Circuit in Richmond upheld a lower-court ruling that the U.S. Army Corps of Engineers had properly issued a permit allowing Purple Line workers to discharge dredge and fill into wetlands and waterways. The court rejected the opponents’ arguments that the agency hadn’t sufficiently considered a less environmentally damaging bus option that wouldn’t have required widening roads and bridges.

“We cannot conclude that the Corps acted arbitrarily or capriciously in failing to consider … unspecified alternative bus options that the plaintiffs appear to favor,” Judge Barbara Milano Keenan wrote in a 15-page opinion.

The Purple Line’s construction is about 40 percent complete but has limped along since September, when the lead contractor quit over what it said were 2½ years of delays and hundreds of millions of dollars’ worth of cost overruns. Major construction is expected to resume this fall, after a new contractor is hired.

The east-west line will connect communities and Metro stations in Prince George’s and Montgomery counties as the region’s first direct suburb-to-suburb rail link.

During oral arguments in March, Keenan had noted that state and federal transit officials had determined that the bus options would be “very slow” and wouldn’t meet the state’s goal of creating a “more efficient” transit system.

In her ruling, she said the Army Corps of Engineers properly determined that a light-rail line was the “least environmentally damaging, practicable alternative” because it had a “much lower impact” on wetlands than other options studied, even though it affected more streams than some.

The Purple Line’s construction, she said, would damage about a half-acre of wetlands and 5,100 linear feet of streams.

Keenan was joined in her opinion by Judges A. Marvin Quattlebaum Jr. and Allison Jones Rushing.

The ruling appears to end the Purple Line’s seven-year legal saga.

John M. Fitzgerald, one of the plaintiffs and an environmental lawyer, said they are unlikely to appeal to the Supreme Court. The other plaintiffs were the advocacy group Friends of the Capital Crescent Trail and resident Leonard Scensny.

“We exposed many flaws in the Purple Line, which would cost not only a safe, shaded trail and undisturbed waters, but billions more than improving bus service for nearly equivalent ridership and transit time,” Fitzgerald said in an email.

The Maryland Department of Transportation praised the decision, saying the state “remains focused on advancing construction to deliver this critical transit project, which will provide essential transit connections and improve access for the residents of Maryland.”

Fitzgerald and the trail group had filed two previous lawsuits, starting in 2014.

An initial ruling in the first case focusing on the project’s ridership forecasts delayed the start of construction by almost a year, until mid-2017, but the U.S. Court of Appeals for the District of Columbia Circuit rejected it.

The opponents also lost a second lawsuit, in which they argued the Purple Line would take government funding away from the region’s aging Metro system.

The Purple Line will have stops at four Metro stations but is not part of the Metro system.