U-Bahn Line D is a partially completed north‒south route of the Frankfurt U-Bahn underground rail network, planned since the 1960s. three sections had been constructed and were in use for U-Bahn services, plus a long section operated with trams. As the route is not continuous there are no through services, but instead underground services U1, U4, U8, U9 and tram lines 12, 15, 21 all partially serve some of the stops on the route then transfer to other routes. The planned Ginnheim Curve () extension would run via the University's to fill the missing gap. The most northerly section serving two stations on the new Reidberg settlement opened in 2010 and operates with U8 and U9 underground services. The core northern section between and had been constructed between 1968‒1978 as a branch of the U-Bahn Line A serving and operates with U1 and U8 services. The middle section between the main railway station at Frankfurt (Main) Hauptbahnhof and opened in 2001 as an extension of U-Bahn Line B (U4 service). South of the River Main between the University Hospital and Schwanheim the planned route remains operated by low-floor tram services but is mostly upgraded to light rail design running in its own reservation. Sections The route was originally planned in four parts. The numbering sequence may have reflected the sequence in which the sections were originally planned to be built. Following the development of , the construction priorities shifted. D 0: Ginnheim – Heddernheim In use, used by services U1 and U9. The northern part from the was already part of the very first line in 1968, when the other branches of the Line A were still being used with converted tram vehicles and under tram line numbers. The extension to Ginnheim originated in the 1970s. The line is rotated more than 90 degrees from the Line A, which is only explained by the intention to integrate the section later in the Line D. D I: Hauptbahnhof – Bockenheimer Warte In use, served by U4 services. The section from Frankfurt (Main) Hauptbahnhof, via to was opened in 2001. The line is operated line by U4 services as an extension of the U-Bahn Line B route. D II: Bockenheimer Warte – Ginnheim Planned. construction of the Ginnheim Curve route was planned to occur in the 2020s on a route via the University's new Campus Westend. Several routes to fill in the