Bicycles, rarely seen in the parking lot at Sandcastle Waterpark, were everywhere on Tuesday as riders celebrated the deal that will complete a trail network from Pittsburgh to Washington, D.C.

The park's property was the last piece needed for the Great Allegheny Passage, and for the longest time, securing the owner's permission was like pumping air into a punctured tire.

"A lot of you thought we'd never get to this day," Allegheny County Executive Dan Onorato said at a ceremonial signing of an easement allowing construction of the last 0.85-mile trail segment. "You had a right to be skeptical."

Years of resistance by Sandcastle's operators melted away after the park was taken over by Palace Entertainment, a California-based subsidiary of Parques Reunidos.

"The new ownership really picked up the ball," Mr. Onorato said. "They had a lot of things to worry about just running their business. They kept us as a priority."

The agreement likely dissolved much of the ire in the region's burgeoning cycling community toward the park.

Darla Cravotta, the county's special projects coordinator, jokingly suggested that park general manager Damian Dondero try to sell season passes to the assembled cyclists.

"This is a crowd that hasn't been purchasing," she said to a wave of laughter.

"We're looking forward to all the bikers that are coming up and down the trail," Mr. Dondero said. "I'm looking forward to putting more bike racks at the entrance."

The ceremony was a milestone in the county's four-year effort to fill the last nine unfinished miles on the 150-mile Great Allegheny Passage, which connects in Cumberland, Md., with the C&O Towpath to Washington.

To do so, trail developers had to find a way across the Monongahela River at McKeesport, over busy freight railroad lines in Duquesne and Whitaker and through Sandcastle, which improbably turned out to be the hardest piece.

"There have been many miracles needed to be accomplished in this last nine miles," said Linda McKenna Boxx, president of the Allegheny Trail Alliance, one of the trail's developers. "We have a lot of work ahead of us to get the trail done by 11-11-11," the county's Nov. 11, 2011, target date, she said.

Ms. Boxx said contributions have been arriving from all over the U.S., from visitors who rode the trail. "People all over want this trail to be done," she said. "This is a true testament to 'If you build it, they will come.' "

Cindy Dunn, deputy secretary of the state Department of Conservation and Natural Resources, said the trail attracted 750,000 visitors and generated $40 million in spending last year.

She suggested that trail advocates not rest on their saddles.

"On 1-1-12 let's stand at the Point and look north," she said. "Let's run the Great Allegheny Passage to Erie and the Pennsylvania Wilds."