missing gap have been proposed. A previous plan to link the two existing sections had been planned to start in 2006 using a route completely in tunnel. One station would have been constructed at Franz-Rücker-Allee and the existing Ginnheim U-Bahn station relocated to below the Ginnheimer Landstraße. Just before construction would have started, the measure was halted owing to changes in political majorities resulting from formation of a new coalition after the local elections of 2005. D III: Hauptbahnhof – Niederrad Incomplete and likely to remain operating as a tram line. It was originally planned to run southwards from Frankfurt Hauptbahnhof, under the River Main to Niederrad and then on to Schwanheim or Frankfurt Stadion station. the only visible construction is an extra unused platform and a pair of stub tunnels heading southwards at the main railway station U-Bahn level. The extra platform is only connected to track in the direction of Bockenheimer Warte and cannot be used in normal service. This platform and the longer of the stub tunnels are used as the Firefighting Practice Tunnel () by the Frankfurt Fire Brigade. Sections of the planned route via and had been partially upgraded in the 1970s in anticipation of conversion to light rail operation. Instead this route and its branches remain operated by tram services. D IV: Riedberg extension This section connects the stations Niederursel (on the Oberurseler branch of the A-line) and Kalbach (on the Bad Homburger branch of the A-line) with each other, thereby opening up the new housing area Riedberg. Two new stations were created in this section: University Campus Riedberg and Riedberg, the latter near the Nelly Sachs Platz. Larger work was an incision in the slope through which leads directly north of the station Niederursel the route to a new bridge to be built on the Rosa-Luxemburg-Straße and the bridge over the A 661 motorway immediately north of the junction Heddernheim. In this connection, a connection between the stations Heddernheimer Landstraße and Wiesenau was established as a connection to the rest of the D line. For this, the existing route from the north portal of the northwest tunnel to the branch towards Heddernheim was swung slightly to the east, whereby the station Heddernheimer highway was shifted slightly to the east and was rebuilt with 80 cm high platforms. Since October 25, 2010 took place on the new track test drives. Two lines went on