If, as Wallace Stegner said, the national park is "the best idea we ever had," the rail-trail is certainly a close runner-up. Part transportation corridor, part park, the rail-trail has revolutionized the way America creates high-quality, car-free pathways for bicyclists, runners, walkers, equestrians, and more. It was only a few decades after railroad barons had run roughshod over America's economy and politics that they began to shed nearly...