As a comment on a recent post points out, there are two cities called Frankfurt in Germany. One is in the west on the river Main and the other is on the river Oder. That river was part of the Oder- Neiße Line, as the post-war border between Germany and Poland was known before it was formally recognised.
When I lived in Offenbach am Main in the late 1970s I saw a train that was going from one Frankfurt to the other.
Frankfurt is the place where the Franks forded the river. The nightlife district if Frankfurt am Main is called Sachsenhausen. That’s where the Saxons had their houses.