In 1967 the Bonn City Council approved a new transportation plan which included underground railway (U-Bahn) between Bonn and Bad Godesberg; the initial ground breaking took place in October of the same year. The overall concept involved a mixed system of trams (operated by the SSB) and heavy rail (operated by the KBE). However, the KBE went into financial decline at the end of the decade and survived only with the support of the North Rhine-Westphalia government; in this environment expansion was impossible. Instead, the railway lines of the KBE would be used to provide a link between Bonn and the Cologne tram network. To this end, new, lighter rolling stock was introduced that would be more suitable on Cologne's Rhine bridges and subway tunnels. The Siegburg lines had already switched to Stadtbahn rolling stock in 1974. On 22 March 1975, the Bonn Stadtbahn was officially inaugurated.