Melbourne is the only major city in Australia with an extensive, street-running tram network that has operated continuously since the 19th century. In fact, it is the largest operational tram network in the world, spanning over 250 km (155 miles) of track with more than 500 trams in service. While cities like Sydney, Adelaide, and the Gold Coast have recently built modern light rail systems, Melbourne's system is unique for its historical integration into the city's character and its massive scale. The city even offers a "Free Tram Zone" within the Central Business District (CBD) and a historic "City Circle Tram" (Route 35) that uses vintage W-class trams to provide free sightseeing for tourists. For Melburnians, the "ding-ding" of the tram is the city's signature sound, and the network remains the primary mode of transport for millions of commuters and visitors annually.