What else can we elevate as parkland?

New York just paid $150 million for something Chicago has acquired through neglect: a green space that winds through the city on the viaduct of an abandoned elevated railway:
A different kind of park opens Tuesday in New York City. The High Line is on top of an old elevated railroad trestle that runs up the West Side of Manhattan. When complete, the park will occupy the 1.5 miles that the rail line used.
Thirty feet above the old Meatpacking District, it’s a different world from the street below. On the narrow iron structure, there is now a park promenade that winds its way through wildflowers and remnants of the old railroad tracks.
The elevated railroad was built in the 1930s to serve the industrial neighborhood. The park follows the same path — over streets and even tunneling through buildings.
via NPR.
In fairness, New York paid its millions mostly to clean up lead paint and reinforce trestles, which made the park safe to be a park. Chicago’s version is not officially a park. You have to shimmy up a tree or clamber up a wall to get to it, and once there you have to beware of trolls and other urban hazards. New York’s park has something called a “promenade.” We don’t have any such animal on the South Side and if we did, we sure wouldn’t call it that.
But the architect of New York’s gem told NPR he tried to make The High Line appear as natural as possible, and Chicago’s version simply is natural. You may not notice it if you’re passing at street level, but if you ride Chicago’s Green Line on the South Side you can’t miss the lingering embankment of the perpendicular Kenwood Line, which once carried workers from lake shore neighborhoods past White Sox Park to the Stockyards. Abandoned since 1957, it now hosts an elevated forest.
Chicago being Chicago, however, the bulldozers will one day come, for the city is always, as Carl Sandburg wrote, “shoveling, wrecking, planning, building, breaking, rebuilding.” Parts of the old Kenwood Line are already gone, including the bridges that once spanned roads, while New York has preserved a continuous line for the generations. Three years ago Chicago razed another forest that had flourished for decades on unused land along the South Branch of the Chicago River, just South of Roosevelt Road. Why? Why not, was the thinking. Can we use New York’s example to encourage not just Chicago, but all cities, to let new parks flourish in old places?
Where else can we plant grass and trees to preserve something for the generations? What weedy waste in your hometown could be a new park?