I love this fabulous idea from New York to "Turn Old Rail Line Into a Park":
The Bloomberg administration moved ahead yesterday with its plans to transform an abandoned elevated rail line into a 1.6-mile-long park and make it the centerpiece for new commercial and residential developments along the western edge of Chelsea.
This article from Boston discusses a similar concept in Paris:
The Viaduc was an abandoned, crumbling, decaying 19th-century railroad viaduct. It was scheduled to be demolished. Instead, it has been transformed into a rich 21st-century combination of shops and parkland. The shops are tucked into the arches that support the viaduct. The park is a strip of green that follows the old train bed, up on top of the arches. You get the life of commerce and the peace of greenery in the same place.
UPDATE: Don't miss this great post by Michael Jennings which includes pictures of the Paris project.
UPDATE 2: And definitely don't miss this one from Social Deign Notes.