This is the Walkway Over the Hudson in Poughkeepsie, a 1.25-mile-long unused railway bridge that was rescued from demolition by open-space-loving activists and transformed into a dramatic midair pathway. Sound familiar? Though its story echoes the High Line, this 120-year-old expanse makes Chelsea’s industrial-reclamation project look puny by comparison: When the Walkway opens on October 3, it’ll be world’s longest pedestrian-only bridge. It also makes a fantastic centerpiece for a fall day trip. Take Metro-North from Grand Central up the Hudson ($29 round trip; 100 minutes each way). Bring picnic provisions (or buy some at Rossi’s Deli two blocks east of the station), then cab or bike ten minutes to the bridge’s entrance on Parker Avenue. You can wind up in Iorio Park and the Hudson Valley Rail Trail on the walkway’s west side, or turn your scenic autumn stroll into a more strenuous hike, looping three miles back to the station via the Mid-Hudson Bridge (walkway.org).