S-trains

The S-train is a type of hybrid urban-suburban rail serving a metropolitan region. Some of the larger S-train systems provide service similar to rapid transit systems, while smaller ones often resemble commuter or even regional rail. They are especially common in Germany, where they are known as S-Bahn, which in the 1930s was an abbreviation of either Schnellbahn, Stadtbahn or Stadtschnellbahn, depending on the city, but they must not be confused with U-Stadtbahns. 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).

There is no complete definition of an S-train system. S-trains are, where they exist, the most local type of railway stopping at all existing stations inside and around a city. They are slower than mainline railways but usually serve as fast crosstown services within the city. Most S-train systems are entirely built on older local railways, or in some cases parallel to an existing dual track railway. Most use existing local mainline railway trackage, but a few branches can be purpose built S-train lines. S-trains typically use overhead lines or a third rail for traction power. In Hamburg the S-trains use both the methods, depending on which line is powered.

In smaller S-train systems and suburban sections of larger ones S-trains typically share tracks other rail traffic, with the Berlin S-Bahn, Hamburg S-Bahn and Copenhagen S-train being notable exceptions. Busy S-train corridors sometimes have sections of exclusive trackage of their own but parallel to mainline railways. A good example of this is the part of Berlin's S-Bahn, which is regarded as a tourist attraction and has a special name, Berliner Stadtbahn. However, in more lightly used sections outside the city center, S-trains share tracks with other train types. The S-trains stop at all stations, while other mainline trains only stop at the largest stations (typically junction stations). S-trains are generally service the hinterland of 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, a large urban agglomeration, not unlike the large net of regional 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 is an example of a smaller S-Bahn system.

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