Seven of these systems are vestigial "legacy" first-generation streetcar systems that were spared the fate of the other streetcar systems that were closed in the United States during the 1950s-1970s due to their having some grade separation from other traffic (e.g. subway tunnels) and relatively high ridership. The New Orleans streetcar system, established in 1835, is the oldest continuously operating street railroad in the world. Many lines would be converted to bus service after 1924. However, several lines have remained unaltered from their initial alignments, and some expansions of the system have been performed. Boston's Tremont Street subway was the first rapid transit tunnel built in North America in 1897, and was intended to speed up service by removing streetcars from the traffic-clogged streets. That tunnel would later see modern light rail service in the form of the Green Line light rail network. The Ashmont–Mattapan High-Speed Line was converted from a commuter rail line in 1929, and has continuously used PCC streetcars since. Cleveland and Philadelphia have light rail systems originally built as interurban railroads that only received maintenance and rolling stock improvements while offering nearly the same services since their inceptions. Additionally, Philadelphia's system of subway–surface streetcar lines, which utilize a tunnel in the downtown area, were largely established in their current configuration by 1956. Newark Light Rail was only fully converted from PCC streetcars to modern light rail vehicles in 2001, having been operating since the 1930s.