Premetro

Premetro (sometimes pre-metro) is a type of light rail transit. Here infrastructure is gradually built with the goal to use rapid transit trains in the distant future, but using trams or light rail vehicles preliminary in the meantime. This infrastructure consists of tunnels and/or viaducts, so vehicles have no conflicts with other traffic. To achieve that these rapid trains will be able to use the infrastructure, wider curves and lesser grades need to be designed and built.^{[1][2][3][4][5]}

The use of tram vehicles in tunnels originated in the United States in the nineteenth century and was often called "subway–surface line". Later, in the second half of the twentieth century the term "semi-metro" was coined for tram systems with some sections in tunnels and on viaducts. Only when a semi-metro section is designed for later use of heavy rapid transit trains, it falls in the premetro category. The large scale report "Light Rail Transit: A State of the Art Review" describes on page 9 the relationship of premetro systems to broader terms as follows:

Certain design features distinguish these systems from semi-metro or conventional light rail transit systems^[2]

The same distinction is made in glossaries.^[1][6] During the period when tram vehicles are used, the specific line falls into the light rail category.^[7][8]

An early example was the Tremont Street subway (1897) in Boston, today part of the MBTA Green Line. This tunnel was intended solely to reduce streetcar congestion on surface streets, not for later conversion to metro service.^[9] However between 1901 and 1908, two out of four tracks were used for rapid transit service including high platforms.^[10] Several early streetcar tunnels, including the Steinway Tunnel and East Boston Tunnel, were later converted to metro operation. However, the small loading gauge, tight curves, and steep grades of the streetcar tunnels required shorter metro cars than otherwise desirable.^[9] In 1950 Stockholm effectively used a pre-war tramtunnel for its first rapid transit line.^[11]

The modern premetro concept Stadtbahn began in 1960s Germany, as rising traffic congestion due to auto ownership led to the construction of new transit systems. Rather than building costly metro lines immediately, some cities built only the downtown tunnels. They could be used by existing tram lines in the short term, with the intention of full metro conversion later - hence "pre-metro".^[12] The idea spread to other European countries in the 1970s, especially Belgium, where such systems were explicitly named premetros.^[12] Also one segment of Vienna's U2 metro line (Rathaus-Museumsquartier) is an in 1980 converted underground tramway line, which was constructed in 1966.

• Line 2 (Rio de Janeiro) in Brazil^[13]
• Two stations of the line Roma – Fiuggi – Frosinone were converted to metro standards to become part of the Rome Line C.
• Vienna Pre-metro in Austria

• Brussels Pre-metro^[14] in Belgium

• Antwerp Pre-metro^[15] in Belgium
• Metrotram in Kryvyi Rih

• Medium-capacity rail system
• Train categories in Europe