S-Bahn

S-Bahn ( , German:; lit.'S-train') is a type of hybrid commuter rail and rapid transit service, originating in German-speaking countries. In general, S-Bahn systems link suburbs with the city centre at moderate speeds, while continuing across the urban core over a central high-frequency corridor where multiple lines converge, where they provide a rapid means of travel across the city. The name S-Bahn derives from the German terms Schnellbahn (lit.'rapid train'), Stadtschnellbahn (lit.'rapid city train'), and Stadtbahn (lit.'city train'; not to be confused with the modern Stadtbahn).

The first such network, the Berlin S-Bahn, began operation in December 1930 and inspired similar systems across Europe. The term "S-Bahn" has since become a generic term for hybrid suburban–urban rail networks, with counterparts such as the S‑tog in Denmark, S‑Trein/TrainS in Belgium, Esko/S‑lines in the Czech Republic, LineeS in Italy, and SKM in Poland. The S-Bahn is also recognized as a train category in several European countries. The concept also influenced systems outside the German-speaking world, including the Réseau Express Régional (RER) in Paris and its counterparts in other cities.

S-Bahn network design principles are also present in several regional rail and commuter rail systems in the English speaking world primary in Australia and New Zealand. The commuter rail systems of Auckland and of Australia's three largest state capitals (Sydney, Melbourne, and Brisbane) all have city centre viaducts or tunnels which enable through-running service for electrified suburban trains. In the US, SEPTA Regional Rail in Philadelphia is similar to an S-Bahn, using a city centre tunnel to enable extensive through-running. The San Francisco Bay Area's BART also uses a similar trunk-and-branches design with the Transbay Tube forming part of an interlined high-frequency trunk between West Oakland and Daly City.

Sydney and Philadelphia have also adopted a similar route naming system to European S-Bahn systems by using a letter (“T” in Sydney and “R” in Philadelphia) and a number to represent each line. Philadelphia dropped this naming system in 2010 in favour of giving each line the name of its terminal station.