Line U6, which had been the Gürtel line of the Stadtbahn, was left mostly unchanged. The station architecture by Otto Wagner was preserved wherever possible. Only the signalling and electric components have been updated, stations and viaducts have been renovated, and the travel direction was changed from left-hand to right-hand running. A complete conversion of all stations and track to U-Bahn would have been too expensive considering the limited loading gauge offered by the historic lines. The class E6 and c6 Stadtbahn rolling stock remained in service but was phased out by the end of the year 2008 and has since 1995 been complemented and finally replaced by new class T and T1 low-floor rolling stock.