The largest metropolitan areas
(Berlin, Hamburg, Munich, Frankfurt-Mainz-Wiesbaden, Stuttgart, Cologne-Düsseldorf-Ruhr District, Nuremburg, Dresden, Leipzig-Halle, Mannheim-Heidelberg-Karlsruhe, Magdeburg, and Rostock)
have a brilliant suburban train system called the S-Bahn.
These are express trains running from the central city deep into the suburbs.
S-Bahn routes primarily run above ground except in the central city where they frequently are underground. Service is fairly frequent, usually 20-30 minutes during off-peak periods. Besides providing suburban service, the S-Bahn also makes several stops in the central city area as well. These stops are generally further apart than those on the U-Bahn or Stadtbahn and therefore makes the S-Bahn a good option for longer central city journeys.
Unlike the other systems above, which are operated by local governments or franchises,
S-Bahn systems are all operated by GermanRail.
As a result, you can use GermanRail tickets on them and S-Bahn lines are typically numbered with an "S" followed by a number (e.g. S1). S-Bahn stations in all cities in Germany are marked with the standard "S" sign shown to the right below.