>>1411226The system arose from a network not unlike MBTA commuter rail. As you can see on this map, there is no tunnel connection between the central and eastern stations, and thus no possibility to run S-Bahnen on a central trunk route. If you were coming in from the west but wanted to go to the eastern parts of the city, you had to change at the central station - either to a tram or to another "Vorortbahn" (suburban rail) that could have taken you to the eastern station and beyond.
Frankfurt, München, and most recently, Leipzig, all have in common that they built S-Bahn tunnels underneath their city centers to connect two or more railway terminuses on separate ends of the city. The prototype for this is the "Stadtbahn" or city railway in Berlin, which was built between Charlottenburg and Berlin's eastern station in 1882.
