In a wind-whipped setting high above the Schuylkill, officials shook hands Thursday over a plan to link the region's trail system by running a pathway across the old Manayunk railroad bridge, creating a "park in the sky."

"This is a fantastic opportunity we have to personally, economically, and physically unite the city with the suburbs," Mayor Nutter said.

The ceremony at midspan launched a $1.5 million project that when complete - estimated at 18 months from now - will use the bridge as a greenway for hikers, bikers, and joggers.

The bridge will connect the Cynwyd Heritage Trail in Lower Merion Township with the Ivy Ridge Trail in Philadelphia. Someday, planners project, it will be possible to get on a bike in the city and ride by trail to the farthest reaches of the suburbs, or the reverse.

"Think of it as a large puzzle being pieced together over time," said Sarah Clark Stuart, campaign director of the Bicycle Coalition of Greater Philadelphia.

No other viaduct here has been retooled for recreation as was done to the mile-long High Line in New York. In that project, a footpath was placed on a former elevated railroad spur along the lower west side of Manhattan; it has been planted as an aerial park.

Montgomery County Commissioner Joseph M. Hoeffel 3d said he felt the local project might eclipse even New York's rendition.

"High Line, schmi-line," he said. "They'll all be talking about the Manayunk Bridge, across the country."

The idea came from Nutter, who reached out to officials in the township and county. A series of meetings with private and public planners was held to craft the concept and procure the funding.

Of the $1.5 million, $1.3 million in federal money for construction will come through the Pennsylvania Community Transportation Initiative. The rest is being furnished by the Delaware Valley Regional Planning Commission and the William Penn Foundation.

The walking and bicycling path across the bridge will connect Lower Merion's recently completed trail with the site of a former SEPTA station at Dupont and High Streets in Manayunk. From there, the trail will be extended on the city side to the Ivy Ridge station. Eventually, it will join the Schuylkill River Trail at Shawmont Avenue.

On the suburban side, the Cynwyd trail starts at the Cynwyd station and parallels the old Pennsylvania Railroad bed. It follows the border between Westminster and West Laurel Hill Cemeteries as it descends toward the river.

Planners are working to create a trail along Lower Merion's border at the river's western edge. It will extend from West Conshohocken to the City Avenue bridge.

Preliminary designs for the "park in the sky" show a trail winding among low shrubs and bright flowering plants. There are tall, transparent fencelike barriers to keep hikers from going too near the edge.

The Manayunk Bridge is an S-shape structure sweeping across the Schuylkill. It dates to 1918. SEPTA acquired the bridge in 1976 to carry a rail line, but shut it down in 1986 due to disrepair. In 1999, SEPTA began stabilizing the bridge; it is currently sound.

SEPTA manager Joseph Casey told the 100 onlookers and officials gathered on the span that the transit agency had no plans to restore commuter service.

"I'm pleased that the use of our bridge will benefit citizens for years and years of enjoyment," he said.