The High Line is a railroad viaduct in Manhattan that was built in the 1930s as a part of Robert Moses's West Side Improvement project. Although the High Line did not become the lifeline of the city it was expected to and has been partially torn down over the years, some trains ran on portions of the track as late as 1980. The necessity of freight train service through Manhattan was gradually usurped by trucking.
Running a mile and a half along the Hudson River from 34th Street through the meat packing district, it is still partially accessible. At points it appears to run directly into brick walls; this is because it was designed for elevated freight trains to run directly through or just beside some of the buildings it served (some factories and warehouses were built around the track.) -- such buildings have since been sealed around it.
Marvelously forgotten and mostly ignored, it's a fascinating day trip for intrepid hikers or urban explorers.
There is some debate concerning the future of the crumbling structure. The problem of the High Line challenges urban planners because if not dealt with it's likely to deteriorate dangerously. One organization is advocating renovating it into a greenway as a part of the federal rails-to-trails program. Other possible solutions are to reestablish a freight railroad, convert it to a light rail line, integrate it into the New York Subway system, or just simply raze the remaining structure.