S-Bahn is an abbreviation of Schnellbahn or Stadtschnellbahn (city rapid rail), and, as the name suggests, is the fastest form of public transportation. It is a kind of urban-suburban rail system and serves a wider metropolitan region, linking the suburbs and commuter regions with the city centre and main rail station.

The term "S-Bahn" is an abbreviation for the German word Stadtschnellbahn, which translates literally to "city fast railway." Introduced in Berlin in 1930, the "S" was chosen to represent "Stadtschnellbahn," though some historically associated it with "Stadtbahn" (city railway) or "Schnellbahn" (fast railway). The "S" branding was designed to distinguish these suburban-inner city hybrid trains from the U-Bahn (Untergrundbahn), which is the traditional underground subway. While the U-Bahn typically stays within the city core and runs primarily underground, the S-Bahn connects the city center with its outlying suburbs, often running on elevated tracks or repurposed mainline heavy rail tracks. The iconic white "S" in a green circle is now a ubiquitous symbol of urban transit across Germany, Austria, and Switzerland. It signifies a service that is faster than a tram or bus but more frequent and local than a regional or intercity train, serving as the critical "connective tissue" of the metropolitan public transport network.