September 30 in Swedish History
1933: The first underground subway tunnel is inaugurated in Stockholm. The decision to build the subway system had been made two years earlier when work also started to build the tunnel under Södermalm, the Stockholm south. The tunnel went from Slussen to Skanstull and was at first run by regular trams.
Signs at the three stations, Slussen, "Södra Bantorget" (now Medborgarplatsen and "Ringvägen" (now Skanstull) advertised with a big T with Tunnel on one side and Banan (Line) on the other, thus creating the Swedish word for subway; Tunnelbanan.

The first real subway line opened 17 years later, in 1950: October 1 in Swedish History