The S-Bahn is a suburban metro railway network in Germany, Austria and Switzerland. The name is an abbreviation for "Stadtschnellbahn" (fast city train) and was introduced in December 1930 in Berlin, after "SS-Bahn" (Stadtschnell...) had been unofficially in use already. The S-Bahn is a railway that serves metropolitan traffic as well as direct regional traffic, and is characterised by high efficiency and a synchronised timetable that allows for denser train traffic on the rail lines. This is achieved by the use of separate tracks, electric locomotives and train doors at platform level (in some cities these standards are not met though). The symbol for the S-Bahn in Germany is a white "S" on a green circle. In Austria S-Bahn lines and stations are displayed by a blue circle with a white, cornered 'S' in it. There are proper S-Bahn Systems in Vienna and Salzburg. The Viennese System is very large, old and well-known. In Switzerland, S-Linien (S-lines) is displayed in black letters on a white background. From the term S-Bahn come many similar notations, like the name R-bahn for regional trains, which do not meet S-Bahn criteria.
The term "RER" (Réseau express régional), used in France and western Switzerland, and the "S-tog" in Copenhagen are very similar to the S-Bahn. In the United Kingdom, Merseyrail (Liverpool) and the SPT Rail Network (Glasgow) are roughly equivalent to S-Bahns. The closest thing London has to an S-Bahn is Silverlink Metro. However, this only covers a small part of London's suburban services, as, owing to the nature of London's railways, they are split across several Train Operating Companies and Termini. Additionally, London's Overground Network in South London tends to function with the frequency of an S-bahn.
S-Bahn lines are different from U-Bahn lines in that they have developed from conventional railways. To this day the German and Austrian S-Bahn lines are operated by subsidiaries of Deutsche Bahn and ÖBB respectively — the national railway organisations. Compared to underground lines, the S-Bahn lines have numerous branches. The lines of different branches operate on one or several stretches together where they create dense train traffic.
In comparison to regional trains, the S-Bahn operates at a higher frequency, but also shorter distance between stations.