page_title: Rapid transit
section_title: Tunnels
hierarchical_section_title: Rapid transit / Infrastructure / Tunnels
caption_attribution_description: en: Description: Sportivnaya metro station, St. Petersburg, Russia. Built in 1997, this two-level Ancient Greek themed station became one of the most beautiful in St. Petersburg metro Author: Paranoid Date: unknown Licence: public domain de: Beschreibung: Die St. Petersburger Metrostation Sportiwnaja Autor: Paranoid Datum: unbekannt Lizenz: public domain / gemeinfrei
context_page_description: Rapid transit or mass rapid transit, also known as heavy rail, metro, subway, tube, U-Bahn, metropolitana or underground, is a type of high-capacity public transport generally found in urban areas. Unlike buses or trams, rapid transit systems are electric railways that operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles of any sort, and which is often grade-separated in tunnels or on elevated railways.
  Modern service on rapid transit systems are provided on designated lines between stations typically using electric multiple units on rail tracks, although some systems use guided rubber tires, magnetic levitation, or monorail. The stations typically have high platforms, without steps inside the trains, requiring custom-made trains in order to minimize gaps between train and platform. They are typically integrated with other public transport and often operated by the same public transport authorities. However, some rapid transit systems have at-grade intersections between a rapid transit line and a road or between two rapid transit lines.
context_section_description: Underground tunnels move traffic away from street level, avoiding delays caused by traffic congestion and leaving more land available for buildings and other uses. In areas of high land prices and dense land use, tunnels may be the only economic route for mass transportation.  Cut-and-cover tunnels are constructed by digging up city streets, which are then rebuilt over the tunnel; alternatively, tunnel-boring machines can be used to dig deep-bore tunnels that lie further down in bedrock.
  The construction of an underground metro is an expensive project and is often carried out over a number of years. There are several different methods of building underground lines.
  In one common method, known as cut-and-cover the city streets are excavated and a tunnel structure strong enough to support the road above is built in the trench, which is then filled in and the roadway rebuilt. This method often involves extensive relocation of utilities commonly buried not far below street level – particularly power and telephone wiring, water and gas mains, and sewers. This relocation must be done carefully, as according to documentaries from the National Geographic Society, one of the causes of the April 22, 1992, explosions in Guadalajara was a mislocated water pipeline. The structures are typically made of concrete, perhaps with structural columns of steel; in the oldest systems, brick, and cast iron were used. Cut-and-cover construction can take so long that it is often necessary to build a temporary roadbed while construction is going on underneath, in order to avoid closing main streets for long periods of time.
  Another usual type of tunneling method is called bored tunneling. Here, construction starts with a vertical shaft from which tunnels are horizontally dug, often with a tunneling shield, thus avoiding almost any disturbance to existing streets, buildings, and utilities. But problems with ground water are more likely, and tunneling through native bedrock may require blasting. The first city to extensively use deep tunneling was London, where a thick sedimentary layer of clay largely avoids both problems. The confined space in the tunnel also limits the machinery that can be used, but specialized tunnel-boring machines are now available to overcome this challenge.
  One disadvantage with this, however, is that the cost of tunneling is much higher than building cut-and-cover systems, at-grade or elevated. Early tunneling machines could not make tunnels large enough for conventional railway equipment, necessitating special low, round trains, such as are still used by most of the London Underground, which cannot install air conditioning on most of its lines because the amount of empty space between the trains and tunnel walls is so small. Other lines were built with cut-and-cover and have since been equipped with air-conditioned trains.
  The deepest metro system in the world was built in St. Petersburg, Russia where in the marshland, stable soil starts more than 50 metres (160 ft) deep. Above that level, the soil mostly consists of water-bearing finely dispersed sand. Because of this, only three stations out of nearly 60 are built near ground level and three more above the ground. Some stations and tunnels lie as deep as 100–120 metres (330–390 ft) below the surface. However, the location of the world's deepest station is not clear. Usually, the vertical distance between the ground level and the rail is used to represent the depth. Among the possible candidates are:
  Deepest stations in Saint Petersburg Metro, Russia:
  Admiralteyskaya (The Admiralty, 102 metres (335 ft), opened 2011, probably the best candidate)
  Komendantsky Prospekt (The Commandant Avenue, 78 metres (256 ft), opened 2005)
  Chernyshevskaya (Chernyshevsky, 70 metres (230 ft), opened 1958)
  Ploshad Lenina (Lenin Square, 72 metres (236 ft), opened 1958)
  Arsenalna station in Kiev Metro, Ukraine (105.5 metres (346 ft), opened 1960, built under a hill)
  Hongtudi station in Chongqing Metro, China (94 metres (308 ft), opened in 2016)
  Liyuchi station in Chongqing Metro, China (76 me
caption: The Sportivnaya station of the Saint Petersburg, Russia metro depicts Ancient Greece; the word "sportivnaya" means "sporty" or "athletic".
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