To relieve the Frankfurt center of too much crowd, Frankfurt will build a S-Bahn ring under the city. Currently Frankfurt's railway system is more like a star system with the main station in the center of the star. If you want to go anywhere, you first have to come to the city center in most cases.
This is what the planners want to change. After the railway pass to the west of the city, now the plans are shaping also for building railways in other directions of the ring.
According to initial plans, a feasibility study for the tunnel ring under the city should be ready in 2021 and the tunnel itself should be operational by the end of the 2030s.
FNP link below gives the details of the tangents, along with their routes and some implementation plans. I do not want to plagiarize their reporting by translating it. It is in German, sorry. Maybe you have a German colleague who could translate for you.
Noch ist es Zukunftsmusik, aber der Schienenring um Frankfurt nimmt langsam Konturen an. Nach der Westtangente soll 2030 auch der Osten und Süden beschleunigt werden.
https://www.fnp.de/frankfurt/frankfurt-bahn-ring-sbahn-hauptbahnhof-stadtteil-tunnel-neu-sued-nord-13904680.html
Konkrete Pläne für einen neuen Bahn-Ring um Frankfurt – Tunnel unter Stadtteil geplant
www.fnp.deNoch ist es Zukunftsmusik, aber der Schienenring um Frankfurt nimmt langsam Konturen an. Nach der Westtangente soll 2030 auch der Osten und Süden beschleunigt werden.