books.google.co.jp - Early in the 19th century, growing American cities began to experience transportation problems. One solution was the horse-drawn streetcar, developed in 1832, but it soon proved inadequate. The first elevated train was transporting passengers above the streets of Manhattan by 1871; the first subway opened...http://books.google.co.jp/books/about/Metropolitan_railways.html?id=foSEPTUE7NMC&utm_source=gb-gplus-shareMetropolitan railways
