Wikipedia
The S-train is a type of electrified hybrid urban and suburban railway. They are especially common in Germany, where they are known as S-Bahn, which originally in the 1930s was an abbreviation of either Schnellbahn, Stadtbahn or Stadtschnellbahn, depending of city, but they must not be confused with the new German U-Stadtbahns of today. Also, the tourist attraction Berliner Stadtbahn as of today is a four track elevated railway, of which two of the tracks is a part of the S-train system in Berlin.
Most S-train systems are entirely built on older local railways or in some cases parallel to an existing dual track railway (alternatively a new S-train branch can take over two of several already existing tracks), but they are never based on old metro tracks or underground fast tram tracks. Most use old local railways, but a few branches were purposely built for S-trains Similar S-train systems exist also in Denmark, there known as S-tog, the Czech Republic as Esko, Switzerland and Austria also as S-Bahn, and northern Italy as Servizio ferroviario followed by either the word "metropolitano" (in Turin) or "surburbano" (in Milano). S-trains are, where they exist, the most local type of railway. Some of the larger S-train systems may be difficult to distinguish from Metro systems. They have always been driven through electricity, but voltage, AC-power or DC-power differs between the cities where they exist. The same applies to powering through hanging lines or a "third track". In Hamburg does the S-trains use both the latter methods, depending of which line that's powered.
The Hamburg S-Bahn and Copenhagen S-train are all S-train systems that use separate, dedicated tracks without level crossings, with passengers access to the platforms without crossing the tracks. The latter also applies to the Vienna S-Bahn, but parts of the Vienna S-Bahn share tracks with other train types outside the city centre. S-trains are generally the most local type of railway where they exist, and usually belong to a certain city, rather than connecting different cities, although in high population density areas a few exceptions from this exist. A good example of a such exception is the Rhine-Ruhr S-Bahn, which interconnects the cities, towns and suburbs of the Ruhr, the large urban agglomeration which contains cities like Dortmund, Essen, Wuppertal and Düsseldorf, unlike the large net of regional trains and local trains which also serve the area. The S-Bahn Mitteldeutschland constitutes the main local railway system for Leipzig but also connects to Halle, where a few stations are located. The Rostock S-Bahn is an example of a smaller S-Bahn system.
S-trains generally stop at all stations along a route with frequent service, with typical weekday service in regular 10 or 15 minute intervals. In many cases, multiple lines serving separate suburban branches will converge into a central trunk line with high combined frequency of all the branches. For instance, on weekdays the Copenhagen S-train serves seven centrally located stations with five lines connecting to and from three main branches, which amounts to 30 trains in each direction. This gives a very high departure frequency through those parts of the city centre area that are home to such stations.
Usually, S-trains serve a similar purpose to a commuter rail. However, a commuter rail tends to focus less on travel within a city's inner boroughs, often in a pattern that radiates out from the city center or runs express through the central area, unlike S-trains that serve both the center city and suburban areas with equal priority. Many larger cities offer S-train service alongside subway, tram, and bus service. Many of the larger S-train systems also have central underground sections that all the individual suburban branches feed into, making these stretches high frequency corridors. Busy S-train corridors sometimes run on their own tracks but parallel to mainline railways. The S-trains stop at all stations, while other trains only stop at the largest stations (typically junction stations).
S-Train (aka Namdo Sea Sightseeing Train is a South Korean tourist train operated by Korail. The train began operations in 2013 and transports tourists through southern South Korea.