History
Stuttgart Stadtbahn, now all standard gauge (1435 mm), has been developed out of a traditional tramway system, which in Stuttgart had metre gauge. In 1961 the city council of Stuttgart decided a general modernization of the municipal tram system; in central areas, the tram should be led underground, in peripheral areas it should get new tracks, separate from road traffic. Intercurrent ideas for a totally new subway were rejected in 1976. At the same time it was decided to enhance modernization of the existing tram in combination with a change from metre gauge to standard gauge. For this reason, Stuttgart made the intermediate effort of rebuilding almost all its tracks with mixed-gauge, on which old streetcars (SSB GT4, built 1959–1965) could run as well as new metro cars (SSB DT 8, in regular service since 1985).
Read more about this topic: Stuttgart Stadtbahn