The first line opened on 19 July 1900, during the 1900 World's Fair ("Exposition Universelle"). The system expanded quickly and the core was complete by the 1920s. Extensions into suburbs were built in the 1930s. The network reached saturation after World War II. The Métro introduced newer trains to allow higher traffic, but further improvements have been limited by the design of the network and in particular the short distances between stations. Besides the Métro, Downtown Paris and its urban area are served by a number of other transit systems.