In 1976–1981, the third-rail Vienna U-Bahn U4 Line substituted the Donaukanallinie and Wientallinie of the "Stadtbahn", built c1900 and first electrified with overhead wires in 1924. This was part of a big project of consolidated U-Bahn network construction. The other electric "Stadtbahn" line, whose conversion into heavy rail stock was rejected, still operates under wires with light rail cars (as U6), though it has been thoroughly modernised and significantly extended. As the platforms on the Gürtellinie were not suitable for raising without much intervention into historic Otto Wagner's station architecture, the line would anyway remain incompatible with the rest of the U-Bahn network. Therefore, an attempt of conversion to third rail would have been pointless. In Vienna, paradoxically, the wires were retained for aesthetic (and economic) reasons.