Streetcars in New Orleans have been an integral part of the city's public transportation network since the first half of the 19th century. The longest of New Orleans' streetcar lines, the St. Charles Avenue Streetcar, is the oldest continuously operating street railway system in the world, according to the American Society of Mechanical Engineers. Today, the streetcars are operated by the New Orleans Regional Transit Authority (RTA). There are currently three operating streetcar lines in New Orleans: The St. Charles Avenue Line, the Riverfront Line, and the Canal Street Line. The St. Charles Avenue Line is the only line that has operated continuously throughout New Orleans' streetcar history (though service was interrupted after Hurricane Katrina in August 2005 and resumed only in part in December 2006, as noted below). The St. Charles Avenue Line has traditionally used streetcars of the type that were common all over the United States in the early parts of the 20th century. Most of the streetcars running on this line are Perley Thomas cars dating from the 1920s. The one exception is a 1890s vintage streetcar that is still in running condition; it is used for maintenance and special purposes. Unlike most North American cities with streetcar systems, New Orleans never adopted PCC cars in the 1930s or 1940s, and never traded in older streetcars for modern light rail vehicles in the later 20th century.