6. Bockenheimer Warte Station – Frankfurt, Germany The Station is one of the most important transfer stations of the Frankfurt subway system. Here the C-line crosses with the U6 and U7, as well as the U4 which runs in the D-tunnel. You can also transfer to various bus lines and trams here. The construction of this station was first begun in 1986, and expanded in 2001. The station is worth viewing not only for its underground architecture, but also for one of its subway entrances.