An S-Bahn is a type of suburban rail network with a bigger footprint than a traditional metro, which links a city to its outer suburbs and inner commuter towns.
The name is an abbreviation of the German: in various cities across the German-speaking world, you’ll find Schnellbahns (“fast trains”), Stadtbahns (“city trains”) or Stadtschnellbahn (“fast city trains”).
But you’ll also find S-tog systems in the Denmark and Esko systems in the Czech Republic. And while no one tends to call it an S-Bahn, the Parisian RER (“Réseau Express Regionale”) is really another version of the same sort of thing: a regional metro system, with largely segregated tracks, on which a number of suburban branch-lines combine to provide frequent services through the city proper.
Source: Some German fella has made a map of London’s “S-Bahn” network, and it’s glorious | CityMetric
