The first line opened without ceremony on 19 July 1900, during the World's Fair (Exposition Universelle). The system expanded quickly until World War I and the core was complete by the 1920s; extensions into suburbs were built in the 1930s. The network reached saturation after World War II with new 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, central Paris and its urban area are served by five RER lines (developed from the 1960s), ten tramway lines (developed from the 1990s) with an additional four under construction, eight Transilien suburban trains, as well as three VAL lines at Charles de Gaulle Airport and Orly Airport. In the late 1990s, Line 14 was put into service to relieve RER A; it reaching Mairie de Saint-Ouen in 2020 constitues the network's most recent extension. A large expansion programme is currently under construction with four new orbital Métro lines (15, 16, 17 and 18) around the Île-de-France region, outside Paris city limits. Other extensions are currently under construction on Line 4, Line 11, Line 12 and Line 14. Further plans exist for Line 1 and Line 10, as does a merger of Line 3bis and Line 7bis.