The term semi-metro refers to a category of urban rail transport in which and/or light rail trains run partly on grade separation tracks to avoid conflicts with other traffic, by using subway and elevated railway.
Dedicated stretches of track are designed to function similarly to regular rapid transit lines.
Systems described as semi-metro run with tram vehicles, as they are usually developed from an existing tram network.
Terminology
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.
In 1962 American transport planner Quinby wrote about a new type of transit, departing from the old "street railway" and reshaping it to form the "limited tramway". Key differences that he mentioned were the development of new, larger rolling stock and the use of grade-separated sections in core urban areas.
More archaic terms that have been used are: subway–surface trolly/streetcar,[[1]]
Features
The term semi-metro refers to systems which sit between rapid transit (with higher investments and a higher capacity and speed) and buses and trams in city streets (with lower investments and a lower capacity and speed). Such systems combine advantages of greater speed (like rapid transit) and better coverage in suburban areas (like buses and trams).
Although cheaper than a metro line, the construction of infrastructure for semi-metro routes is often still too expensive.
Therefore some sections of the line are sometimes constructed later than others, or not at all.
The direct contact 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.
Right-of-way category
A rail transit system is categorised first by its main right-of-way category, and secondly by other parameters like power supply and operating speed.
Three major right-of-way categories can be distinguished, labelled A, B and C by Vuchic:
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Category A: independent right-of-way, without level road crossings or pedestrian crossings resulting in conflict free sections.
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Category B: reserved right-of-way to avoid traffic congestion, but with level road crossings or pedestrian crossings.
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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 means an environment with reduced other modes of traffic.
History
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 building at Grade II by
Historic England in 1998.
Brussels, Cologne and Frankfurt pioneered in Europe with longer tunnels with multiple stations starting in the 1950s, reinventing the idea of the Tremont Street subway.
Subtypes
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
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.
U-Stadtbahn
The
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.
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There are lines where full-sized (i.e. wide) express trams run, with long wagon bodies: Cologne, Frankfurt U-Bahn and Stuttgart, among others.
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In some networks, narrower Stadtbahn trams with shorter wagon bodies were used at the start of the operation: Hannover (TW6000) and Bielefeld (Düwag M/N).
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From the end of the 20th century Stadtbahn lines with low-floor trams also appeared: Dortmund (U43 & U44), Düsseldorf ( Wehrhahnlinie) and Cologne (1, 7, 9, 12 and 15).
Examples
In the United States, the most prominent examples that can be categorised as semi-metro are the San Francisco
Muni Metro and the 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,
In France the networks of Rouen tramway, Strasbourg and Nice tramway are the most prominent ones.
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