More than a decade after a group of urban pioneers living in the Loft District suggested the abandoned Reading Railroad viaduct could be repurposed as an elevated park, an unusual partnership will make the idea a reality and enable construction to start this summer on the first phase of the project.

The William Penn and Knight Foundations plan to formally announce Monday that they are joining forces and checkbooks to fund $11 million worth of park improvements in Philadelphia, primarily in underserved neighborhoods.

Along with providing money to landscape the Reading 'spur' - the long ramp that connects North Broad Street to the main viaduct - the foundation grants will support four other projects that have gripped the public's imagination, including a mile-long portion of the Schuylkill Banks trail and an innovative playground for preteens near the Please Touch Museum.

Though the $11 million donation is by no means record-breaking, especially in comparison with some recent private contributions in New York and Dallas, the foundations have targeted the money to have an immediate impact. All five projects are shovel-ready, have raised most of the necessary construction funds, and can be completed relatively quickly, in less than two years.

Each is in a different part of the city. The hope is that each will become a "civic commons" for its neighborhood, said Carol Coletta, the Knight Foundation vice president involved with the project. The collaboration grew out of a desire to strengthen poorer neighborhoods that have been bypassed by the city's construction boom, she added.

The biggest of the five projects is not the viaduct, but the innovative playground in the Parkside neighborhood. Inspired by the latest theories in "creative play," the $6 million development includes a climbing wall, a mini-mountain range, and a spray park that turns into a winding ice-skating track in winter. Called "Centennial Commons," it will sprawl across the rolling lawn between the Please Touch Museum and the School of the Future on Parkside Avenue. It's being designed by the landscape architect responsible for the popular Sister Cities Park on Logan Square, Studio Bryan Hanes.

The foundations also decided to contribute $250,000 for a new, mile-long segment of the Schuylkill Banks trail north of Bartram's Garden. This is not, as some might expect, the next logical segment of the trail, which now ends at the South Street Bridge.

Instead, the foundations decided to skip across the river to Southwest Philadelphia, near Kingsessing. This section of the trail, called "Bartram's Mile," will start at the garden and wind north through brownfields and housing projects, to about 48th Street. Designed by Andropogon Associates Ltd., it will give residents in that outlying neighborhood better access to the riverfront.

Ultimately, the trail will continue across the river by way of a former railroad bridge and link up the section with the existing sections on the east side. When the trail is completed, residents of Kingsessing will be able to walk, run, or bike into Center City.

The partnership, which is being called "Re-Imagining the Civic Commons," will provide $1.75 million to refurbish the Lovett Memorial Library and Lovett Park on Germantown Avenue in Mount Airy. It was chosen because it straddles the border between the neighborhood's prosperous west side and the less affluent east side.

The foundations are also contributing $1 million to help Audubon and Outward Bound develop a special program for wilderness skills in the nature preserve near the East Fairmount Park reservoir, next to Strawberry Mansion, called the "Discovery Center." Outward Bound is developing a curriculum that uses nature to teach young people leadership.

But there is little doubt that the viaduct park, also designed by Studio Bryan Hanes, will get the most attention. The idea of creating an elevated rail park on the model of New York's High Line has been a topic of intense discussion for years. With the foundations' $1 million grant, Philadelphia will join a select group of cities with high-line parks. The project still needs to tie up a few loose ends before construction can start, according to Paul Levy of the Center City District, which will manage the construction but not run the park.

Rather than giving the money directly to individual parks groups, the foundations decided to funnel their grant through the Fairmount Park Conservancy, a nonprofit that is becoming a clearinghouse for private donations to city parks. After construction is finished, the conservancy will collect data to measure the impact of the new parks on their neighborhoods.

"Philadelphia is serving as a test kitchen," explained Kathryn Ott Lovell, the conservancy's executive director.

After all the construction is complete, all the parks but the Discovery Center will come under the oversight of the city's Department of Parks and Recreation.

The William Penn Foundation has long been the dominant force in the creation of new parks in the city. It spends about $13 million a year to underwrite park design and construction, and is responsible for launching Sister Cities Park, the Race Street Pier, and last summer's Spruce Street Harbor Park.

By partnering with Knight, William Penn hopes to extend its reach even further, said Shawn McCaney, who directs the creative communities program.