Semi-metro (also known as subway-surface[1][2][3][4] line or hybrid[5] streetcar/light rail line) is a form of public rail transport in which trams run partly on a conflict-free track, by using tunnels and/or viaducts.[6] These stretches of track are designed to function like a regular metro or rapid transit line.[7][8] Semi-metro lines run with tram cars because they are usually developed from an existing tram network.[9][10]
One key difference with metro/rapid transit is that semi-metro lines only partially run in tunnels and/or on viaducts.[11] A metro line has an entirely conflict-free track, often completely grade separated. Semi-metro routes are operated by regular trams (with or without low floor) or with specially developed streetcars/tramcars (light rail vehicles), such as the Stadtbahn-car 'type B'.[12]
The term semi-metro falls under the umbrella term light rail,[13] which includes many kinds of modern tram transport. 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 in phases. The entanglement with the existing tram network is an advantage compared to constructing a separate light metro line.[14][15] Often several tram branches at grade are needed in order to make fully use of the high capacity tunnels.[16]
The first city 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. The next prominent example was the Tremont Street subway (1897) in Boston,[17][18] today part of the MBTA Green Line. Brussels, Cologne and Frankfurt pioneered in Europe in the 1960s.
Semi-metro networks can be divided into two subtypes. Both terms refer to tram networks where tram vehicles use viaducts and/or run through tunnels under city centres, but with small differences:
There are many regions with forms of light rail, but only few where light rail uses tunnels and/or viaducts. In the United States, the most prominent examples are the San Francisco Muni Metro and Green Line in Boston.[24][25] The Buffalo Metro Rail, Seattle's Link light rail[26] and the light rail lines in Cleveland[27] are also considered semi-metro.
Notable examples in Germany are the Hanover Stadtbahn,[28] Essen Stadtbahn,[29] Bonn and Cologne Stadtbahns, and the Frankfurt tramways.[25] In the United Kingdom, the Tyne and Wear Metro is by definition a semi-metro system due to eight level crossings.[30] Over several decades a semi-metro system was constructed in the Dutch city of The Hague.[31][32]
More recent examples are the Madrid Metro Ligero, the Málaga Metro and Alicante Metropolitan-Tram in Spain and the Porto Metro in Portugal.[33]