In contrast to the meter-gauge streetcar lines that BOGESTRA had operated since the 1890s, the 1970s and 1980s saw the planning for Stadtbahn lines throughout the Rhine-Ruhr region, including in the BOGESTRA operating area. BOGESTRA's new Stadbahn line would be operated on standard gauge rail with high platform stations in its own right-of-way using Stadtbahn B train cars. The first Stadtbahn section was opened on 2 September 1989 between Bochum Central Station and Castle Herne Strünkede as Line U35. This was also the first inter-city subway tunnel in Germany, because the city boundary between Bochum and Herne is crossed underground on Line U35. According to BOGESTRA, the underground section connecting these cities is still unique in Germany. An extension of Line U35 south from the Bochum Central Station to the Ruhr-University Bochum and on to Hustadt took place on 28 November 1993.