Semi-metro

The term semi-metro refers to a category of urban rail transport in which trams run partly on separate tracks to avoid conflicts with other traffic, by using tunnels and viaducts. This concept is also referred to by various other terms, including subway–surface line/system, tram subway line/system line, trolley subway system or hybrid streetcar/light rail line.
Dedicated stretches of track are designed to function similarly to regular metro or rapid transit lines. Lines described as semi-metro run with tram vehicles as they are usually developed from an existing tram network.
One key difference from metro lines (rapid transit) is that a metro line has an entirely conflict-free track, often completely grade separated, whereas semi-metro describes lines which only partially run in tunnels and on viaducts. Semi-metro routes are operated by regular trams (with or without low floor) or with specially developed tramcars (light rail vehicles), such as the Stadtbahn-car 'type B'. In the United States, most semi-metro systems are operated with larger vehicles than those on streetcar systems.
The term semi-metro can refer to systems which sit between rapid transit (with higher investments and a higher capacity and speed) and buses and tram in city streets (with lower investments and a lower capacity and speed). It combines advantages of greater speed (from rapid transit) with a better coverage in suburban areas (from busses and trams). The term semi-metro falls under the umbrella term term light rail, which includes many kinds of modern transport derived from tram technology. Semi-metro is in itself a container concept in which premetro and Stadtbahn fall. Although cheaper than a metro line, the construction of infrastructure for semi-metro routes was often still too expensive. Therefore sections were sometimes not constructed or realised gradually. The entanglement with the existing tram network is an advantage compared to constructing a separate light metro line. Often several tram branches at grade are needed in order to make fully use of the high capacity tunnels.
A rail transit system is firstly determined by its main right-of-way category and secondly by other parameters like power supply and operating speed. There are three major right-of-way categories, having been labelled A, B and C. Category A: independent right-of-way, without level road crossings or pedestrian crossings resulting in conflict free sections. Category B: reserved right-of-way to avoid traffic congestion, but with level road crossings or pedestrian crossings. Category C: street running lines in mixed traffic. Typically, a tram/streetcar line has mostly category C, a light rail line has mostly category B and a semi-metro line has some of category A (combined with category B and sometimes C).
Alternatively, the Transportation Research Board terms the independent right-of-way as grade separated: including "facilities located at grade, where modes cross over or under the facility". The reserved right-of-way is termed exclusive and a subtype is distinguished: semi-exclusive, which is an environment with reduced other modes of traffic. Whenever light rail vehicles operate only using category A, it is defined as "light rail rapid transit" (LRRT) and is part of the greater light metro class.
An early example that has semi-metro characteristics was the Murray Hill Tunnel in New York, which was completed in 1850, with an underground streetcar station that opened in 1870. Streetcars continued to run until 1935, and the tunnel was reopened as a road tunnel in 1937 after reconstruction. The first city in Europe to carry a portion of a streetcar line through the city center in a tunnel was Marseille, France, in 1893, with its Noailles subterranean station (see Marseille tramway). It was initially operated by horse-drawn wagons. A prominent example is the Tremont Street subway (1897) in Boston, today part of the MBTA Green Line. The Kingsway tramway subway is a cut-and-cover tunnel in central London, built by the London County Council. The subway opened in 1906 and closed in 1952. The southern end was adapted as a traffic underpass in 1964. The separate northern section was listed at Grade II by Historic England in 1998. Brussels, Cologne and Frankfurt pioneered in Europe with longer tunnels with multiple stations in the 1960s.
Besides regular semi-metro networks, two subtypes exist. Both terms refer to tram networks where tram vehicles use viaducts and/or run through tunnels under city centres, but with key differences:
Premetro is largely equal to semi-metro: a type of public transport in which trams run partly grade separated, by using tunnels and/or viaducts. However, there is one clear distinguishing factor: premetro uses infrastructure that has been explicitly constructed with the ambition to transfer to use metro trains in the future. It is usually also developed from an existing classic tram network. One prominent example is the premetro in Brussels, where several premetro lines have been or will be converted into full heavy rail metro lines.
The U-Stadtbahn is also an intermediate transportation form between metro and tram. It has originated in Germany, adapting the existing tram networks. Here specially developed trams run underground through tunnels in central urban areas. Stadtbahn lines can be subdivided by looking at the types of rolling stock.
In the United States, the most prominent examples that can be categorised as semi-metro are the San Francisco Muni Metro and Green Line in Boston. The Buffalo Metro Rail, Seattle's Link light rail, the light rail lines in Cleveland and the MetroLink in St. Louis are also considered semi-metro. A clear example in Canada is the Edmonton LRT. In the United Kingdom, the Tyne and Wear Metro would be, by this definition, a semi-metro system due to eight level crossings.
Notable examples in Germany are the Hanover Stadtbahn, the Stuttgart Stadtbahn the Essen, Bonn and Cologne Stadtbahns plus the system in Frankfurt. Over several decades a semi-metro system was constructed in the Dutch city of The Hague. The Brussels premetro is sometimes also referred to as semi-metro, as the term was originally coined there; this was before switching to the concept of pre-metro. Other networks in Belgium exist in Antwerp and Charleroi.
In France the networks of Rouen, Strasbourg and Nice are the most prominent ones. In Central Europe, the cities Brno, Linz and Graz provide underground sections, just like Vienna where trams also use premetro infrastructure. More recent examples that could be categorised as semi-metro are the Madrid Metro Ligero, the Málaga Metro and Alicante Metropolitan-Tram in Spain and the Porto Metro in Portugal.