The central section between the main railway station and Bockenheimer Warte [de] opened in 2001 and is operated as an extension of the U4 service from the B Line.

[1] South of the River Main between the University Hospital and Schwanheim the planned route of the D Line remains operated by low-floor trams but has mostly been upgraded run on reserved track.

[citation needed] The route of the D Line is divided into four sections, identified with roman numerals: D I (Hauptbahnhof – Bockenheimer Warte, D III (Ginnheim – Heddernheim) and D IV (Riedberg) are in operation.

From the Hauptbahnhof the D Line tunnel heads in a northerly direction, following Düsseldorfer Straße to Platz der Republik.

The platform hall itself is two storeys high and receives natural light through two glass cones at street level at above it.

The extension of the route to Ginnheim, continuation (D II) would connect to the outer tracks and the other two could still be used for by trains terminating at Bockenheimer Warte.

Some of the larger parts of the construction work were the cutting through the slope directly north of the station Niederursel and the new bridges over Rosa-Luxemburg-Straße and the A661.

In connection with this work a triangular junction was built between the stations at Heddernheimer Landstraße and Wiesenau to allow trains from Riedberg to access the section of the D Line towards Ginnheim.

Sections of the planned route via Universitätsklinikum Frankfurt [de] and Niederrad were partially upgraded in the 1970s in anticipation of conversion to light rail operation.

As of 2021[update] 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 tunnel would follow the Main river and reach the existing tram line via a ramp in the area of the University Hospital.

At Triftstraße the line split would into two branches, which largely correspond to the existing tram routes to the stadium and to Schwanheim.

In the case of a switch to light rail operation, only the platforms of the existing stops would have to be expanded to a height of 80 cm (31 in) and a length of 105 metres (344 ft).

Due to protests of the residents, who feared a separation effect of the light rail route, the building was postponed to the time of conversion to U-Bahn.

In the meantime, however, it is planned to implement the new line in the medium term anyway in order to improve the development of the residential area Goldstein.

The branch to the Waldstadion stadium follows the current tram route along Rennbahnstraße and Schwarzwaldstraße south from the junction on Triftstraße.

In the case of the switch to U-Bahn operation extensive reconstruction measures would be required separate the route from other road traffic.