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Subway (rail)

For other uses, see Rapid transit (disambiguation).
"Metro line" redirects here. For the UK company, see Metroline. For the line in Edmonton, Canada, see Metro Line.

Rapid transit is a type of high-capacity public transport generally found in urban areas.[1][2][3] Unlike buses and trams, rapid transit systems operate on an exclusive right of way which is usually grade separated in tunnels or elevated railways.[4]

Modern services 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 tyres, magnetic levitation, or monorail. The stations typically have high platforms, without steps inside the trains, requiring custom-made trains in order to avoid gaps. They are typically integrated with other public transport and often operated by the same public transport authorities, but does not exclude a fully segregated light rail transit. It is unchallenged in its ability to transport large amounts of people quickly over short distances with little use of land. Variations of rapid transit include people movers, small-scale light metro, and the commuter rail hybrid S-Bahn.

The first rapid-transit system was the partially underground Metropolitan Railway which opened in 1863, and now forms part of the London Underground.[5] In 1868, New York opened the elevated West Side and Yonkers Patent Railway, initially a cable-hauled line utilising static steam engines.

The world's largest rapid transit system by both length of routes (including non-revenue track) and number of stations is the New York City Subway; by length of passenger lines, the largest are the Seoul Metropolitan Subway, Beijing Subway, Shanghai Metro[6] and London Underground. The busiest metro systems in the world by daily and annual ridership are the Tokyo subway system, the Seoul Metropolitan Subway, and the Moscow Metro.


Metro is the most common term for underground rapid transit systems.[7] Rapid transit systems may be named after the medium through which their busier inner-city sections travel; the use of tunnels inspires names such as subway,[8] underground,[9] Untergrundbahn (U-Bahn) in German,[10] or Tunnelbana (T-bana) in Swedish;[11] the use of viaducts inspires names such as elevated (el or L), skytrain,[12] overhead, or overground. One of these terms may apply to an entire system, even if a large part of the network (for example, in outer suburbs) runs at ground level.

In most of Britain, a subway is a pedestrian underpass; the terms Underground and Tube are used for the London Underground, and the Tyne and Wear Metro, mostly overground, is known as the Metro. In Scotland, however, the Glasgow Subway underground rapid transit system is known as the Subway.


The opening in 1863 of the steam hauled Metropolitan Railway marked the beginning of rapid transit. Initial experiences with steam engines, despite ventilation, were unpleasant. Experiments with pneumatic railways failed in their extended adoption by cities. Electric traction was more efficient, faster and cleaner than steam and the natural choice for trains running in tunnels and proved superior for elevated services. In 1890 the City & South London Railway in London was the first electric traction rapid transit railway, which was also fully underground.[13] Both railways were eventually merged into London Underground. The 1893 Liverpool Overhead Railway was designed to be electric traction from the outset.[14]

The technology quickly spread to other cities in Europe and the United States with some railways being converted from steam and others being designed to be electric from the outset. Budapest in Hungary and Glasgow, Chicago and New York all converted or purpose designed and built electric rail services.[15] There were 19 systems by 1940, and 66 by 1984. Cities such as Oslo and Marseille opened extensive systems in the 1960s and many new systems were introduced in Southeast Asia and Latin America.[10]

Advancements in technology have allowed new automated services. Hybrid solutions have also evolved, such as tram-train and premetro, which incorporate some of the features of rapid transit systems.[13] In response to cost, engineering considerations and topological challenges some cities have opted to construct trams systems.[16]


Rapid transit is used in cities, agglomerations, and metropolitan areas to transport large numbers of people often short distances at high frequency. The extent of the rapid transit system varies greatly between cities, with several transport strategies.

Some systems may extend only to the limits of the inner city, or to its inner ring of suburbs with trains making frequent station stops. The outer suburbs may then be reached by a separate commuter rail network where more widely spaced stations allow higher speeds. In some cases the differences between urban rapid transit and suburban systems are not clear.[3]

Rapid transit systems may be supplemented by other systems such as buses, trams, or commuter rail. This combination of transit modes serves to offset certain limitations of rapid transit such as limited stops and long walking distances between outside access points. Bus or tram feeder systems transport people to rapid transit stops.[17] In Toronto, over 50% of its rapid transit stations have bus and streetcar terminals within the fare-paid zone, providing a connection without requiring proof of payment.


Each rapid transit system consists of one or more lines. Each line is serviced by at least one specific route with trains stopping at all or some of the line's stations. Most systems operate several routes, and distinguish them by colors, names, numbering, or a combination thereof. Some lines may share track with each other for a portion of their route or operate solely on their own right-of-way. Often a line running through the city center forks into two or more branches in the suburbs, allowing a higher service frequency in the center. This arrangement is used by many systems, such as the Copenhagen Metro[18] and the New York City Subway.[19]

Alternatively, there may be a single central terminal (often shared with the central railway station), or multiple interchange stations between lines in the city centre, for instance in the Prague Metro.[20] The London Underground[21] and Paris Métro[22] are densely built systems with a matrix of crisscrossing lines throughout the cities. The Chicago 'L' has most of its lines converging on The Loop, the main business, financial, and cultural area. Some systems have a circular line around the city center connecting to radially arranged outward lines, such as the Moscow Metro's Koltsevaya Line and Tokyo's Yamanote Line.

The capacity of a line is obtained by multiplying the car capacity, the train length, and the service frequency. Heavy rapid transit trains might have six to twelve cars, while lighter systems may use four or fewer. Cars have a capacity of 100 to 150 passengers, varying with the seated to standing ratio—more standing gives higher capacity. Bilevel cars, used mostly on German S-Bahn type systems, have more space, allowing the higher seated capacity needed on longer journeys. The minimum time interval between trains is shorter for rapid transit than for mainline railways owing to the use of block signaling: the minimum headway might be 90 seconds, which might be limited to 120 seconds to allow for recovery from delays. Typical capacity lines allow 1,200 people per train, giving 36,000 people per hour. The highest attained capacity is 80,000 people per hour by the MTR Corporation in Hong Kong.[23]

Network topologies

Passenger information

Rapid transit operators have often built up strong brands. The use of a single letter as a station sign has become widespread, with systems identified by the letters L, M, S, T and U, among others.[24] Branding has focused on easy recognition—to allow quick identification even in the vast array of signage found in large cities—combined with the desire to communicate speed, safety, and authority.[25]

In many cities, there is a single corporate image for the entire transit authority, but the rapid transit uses its own logo that fits into the profile. In the Singapore MRT, each station was assigned a unique alphanumeric symbol. Interchange stations will then have at least two codes. For example, HarbourFront will have two codes, NE1 for the North East Line section and CC29 for the Circle Line section.

A transit map is a topological map or schematic diagram used to show the routes and stations in a public transport system. The main components are color-coded lines to indicate each line or service, with named icons to indicate stations. Maps may show only rapid transit or also include other modes of public transport.[26]

Transit maps can be found in transit vehicles, on platforms, elsewhere in stations, and in printed timetables. Maps help users understand the interconnections among parts of the system; for example, they show the interchange stations where passengers can transfer between lines. Unlike conventional maps, transit maps are usually not geographically accurate, but emphasize the topological connections among the different stations. The graphic presentation may use straight lines and fixed angles, and often a fixed minimum distance between stations, to simplify the display of the transit network. Often this has the effect of compressing the distance between stations in the outer area of the system, and expanding distances between those close to the center.[26]

Timetables are typically only published if the service frequency is so low that passengers can profitably time their arrival at the station; if the service is frequent enough (say 6 or more trains an hour) passengers will never have to wait long and will not need a timetable.[27]

With widespread use of the Internet and cellphones in many countries, transit operators now use these technologies to present information to their users. In addition to online maps and timetables, some transit operators now offer real-time information which allows passengers to know when the next vehicle will arrive, and expected travel times. The standardized GTFS data format for transit information allows many third-party software developers to produce web and smartphone app programs which give passengers customized updates regarding specific transit lines and stations of interest.

Safety and security

Compared to other modes of transport, rapid transit has a good safety record, with few accidents. Rail transport is subject to strict safety regulations, with requirements for procedure and maintenance to minimize risk. Head-on collisions are rare due to use of double track, and low operating speeds reduce the occurrence and severity of rear-end collisions and derailments. Fire is more of a danger underground, such as the King's Cross fire in London in November 1987, which killed 31 people. Systems are generally built to allow evacuation of trains at many places throughout the system.[28][29] The high platforms (usually over 1 meter/over 3 feet) is a safety risk, as people falling onto the tracks have trouble climbing back.

Rapid transit facilities are public spaces and may suffer from security problems: petty crimes, such as pickpocketing and baggage theft, and more serious violent crimes. Security measures include video surveillance, security guards, and conductors. In some countries a transit police may be established. These security measures are normally integrated with measures to protect revenue by checking that passengers are not travelling without paying.[30] Rapid transit systems have been subject to terrorism with many casualties, such as the 1995 Tokyo subway sarin gas attack[31] and the 2005 7/7 terrorist bombings on the London Underground.


Most rapid transit trains are electric multiple units with lengths from three to over ten cars.[32] Power is commonly delivered by a third rail or by overhead wires. The whole London Underground network uses fourth rail and others use the linear motor for propulsion.[33] Most run on conventional steel railway tracks, although some use rubber tires, such as the Montreal Metro and Mexico City Metro and some lines in the Paris Métro. Rubber tires allow steeper gradients and a softer ride, but have higher maintenance costs and are less energy efficient. They also lose traction when weather conditions are wet or icy, preventing above-ground use of the Montréal Metro but not rubber-tired systems in other cities.[34] Crew sizes have decreased throughout history, with some modern systems now running completely unstaffed trains.[35] Other trains continue to have drivers, even if their only role in normal operation is to open and close the doors of the trains at stations.

Guideway types

Main article: List of metro systems

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.[13]

Street-level railways are used only outside dense areas, since they create a physical barrier that hinders the flow of people and vehicles across their path. This method of construction is the cheapest as long as land values are low. It is often used for new systems in areas that are planned to fill up with buildings after the line is built.[36] Surface-level systems may have dedicated rights-of-way, or may operate by street running in mixed traffic.

Elevated railways are a cheaper and easier way to build an exclusive right-of-way without digging expensive tunnels or creating barriers. In addition to street level railways they may also be the only other feasible alternative due to considerations such as a high water table close to the city surface that raises the cost of, or even precludes underground railways (e.g. Miami). Elevated guideways were popular around the beginning of the 20th century, but fell out of favor; they came back into fashion in the last quarter of the century—often in combination with driverless systems, for instance Vancouver's SkyTrain, London's Docklands Light Railway,[37] the Miami Metrorail, and the Bangkok Skytrain.[38]

People mover systems are self-contained rapid transit systems serving relatively small areas such as airports, downtown (central) districts or theme parks, either as independent systems or as shuttle services feeding other transport systems. They are usually driverless and normally elevated. Monorails have been built as both conventional rapid transits and as people movers, either elevated or underground. They are in commercial use in several places, including Germany, Japan and many international airports.

Light metro is used when the speed of rapid transit is desired, but for smaller passenger numbers. It often has smaller trains, of typically two to four cars, lower frequency and longer distances between stations, though it remains grade separated. Light metros are sometimes used as shuttles feeding into the main rapid transit system.[39] Some systems have been built from scratch, others are former commuter rail or suburban tramway systems that have been upgraded, and often supplemented with an underground or elevated downtown section.[11]


Main article: Metro station

Stations function as hubs to allow passengers to board and disembark from trains. They are also payment checkpoints and allow passengers to transfer between modes of transport, for instance to buses or other trains. Access is provided via either island- or side platforms.[40] Underground stations, especially deep-level ones, increase the overall transport time: long escalator rides to the platforms mean that the stations can become bottlenecks if not adequately built. Some underground stations are integrated into shopping centers, or have underground access to large nearby commercial buildings.[41] In suburbs, there may be a "park and ride" connected to the station.[42]

To allow easy access to the trains, the platform height allows step-free access between platform and train. If the station complies with accessibility standards, it allows both disabled people and those with wheeled baggage easy access to the trains,[43] though if the track is curved there can be a gap between the train and platform. Some stations use platform screen doors to increase safety by preventing people falling onto the tracks, as well as reducing ventilation costs.

The deepest station in the world is Arsenalna station in Kiev, Ukraine.[44]

Particularly in the former Soviet Union and other Eastern European countries, but to an increasing extent elsewhere, the stations were built with splendid decorations such as marble walls, polished granite floors and mosaics—thus exposing the public to art in their everyday life, outside galleries and museums. The systems in Moscow, St. Petersburg, Tashkent and Kiev are widely regarded as some of the most beautiful in the world.[45] Several other cities such as Stockholm, Montreal, Lisbon, Naples and Los Angeles have also focused on art, which may range from decorative wall claddings, to large, flamboyant artistic schemes integrated with station architecture, to displays of ancient artifacts recovered during station construction.[46] It may be possible to profit by attracting more passengers by spending relatively small amounts on grand architecture, art, cleanliness, accessibility, lighting and a feeling of safety.[47]

Modal tradeoffs and interconnections

Main article: Urban rail transit

Since the 1980s trams have incorporated several features of rapid transit: light rail systems (trams) run on their own rights-of-way, thus avoiding congestion; they remain on the same level as buses and cars. Some light rail systems have elevated or underground sections. Both new and upgraded tram systems allow faster speed and higher capacity, and are a cheap alternative to construction of rapid transit, especially in smaller cities.[16]

A premetro design means that an underground rapid transit system is built in the city centre, but only a light rail or tram system in the suburbs. Conversely, other cities have opted to build a full metro in the suburbs, but run trams in city streets to save the cost of expensive tunnels. In North America, interurbans were constructed as street-running suburban trams, without the grade-separation of rapid transit. Premetros also allow a gradual upgrade of existing tramways to rapid transit, thus spreading the investment costs over time. They are most common in Germany with the name Stadtbahn.[32]

Suburban commuter rail is a heavy rail system that operates at a lower frequency than urban rapid transit, with higher average speeds, often only serving one station in each village and town. Commuter rails of some cities (such as German S-Bahns, Chennai rail, Australian cityrails, Danish S-tog etc.) widely provide a mass transit within city as urban metro systems. As opposition, in some cities (such as PATH in New York, Dubai Metro, Los Teques Metro, Tyne & Wear Metro, MetroSur and other lines of Madrid Metro, Singapore MRT, Taipei Metro, Kuala Lumpur's RapidKL Light Rail Transit etc.) the mainly urban rapid transit systems branch out to the nearest suburbs.

Some cities have opted for a hybrid solution, with two tiers of rapid transit: an urban system (such as the Paris Métro, Berlin U-Bahn, London Underground) and a suburban system with lower frequency (such as their counterparts RER, S-Bahn, future Crossrail, respectively). The suburban systems run on their own tracks with high frequency, but are often quite similar to commuter rail, and are often operated by the national railways. In some cities the national railway runs through tunnels in the city centre; sometimes commuter trains have direct transfer to the rapid transit system, on the same or adjoining platforms.[48][49] California's BART system functions as a hybrid of the two: in the suburbs, it functions like a commuter rail, with longer trains, longer intervals, and longer distance between stations; in downtown San Francisco, many lines join and intervals drop to normal subway levels, and stations become closer together. Also, some other urban or "near urban" rapid transit systems (Guangfo Metro, East Rail Line in Hong Kong, Seoul Subway Line 1, etc.) serves the bi- and multi-nucleus agglomerations.

Costs, benefits, and impacts

Main article: Transport planning

As of May 2012, 184 cities have built rapid transit systems.[50] The capital cost is high, as is the risk of cost overrun and benefit shortfall; public financing is normally required. Rapid transit is sometimes seen as an alternative to an extensive road transport system with many motorways;[51] the rapid transit system allows higher capacity with less land use, less environmental impact, and a lower cost.[52]

Elevated or underground systems in city centers allow the transport of people without occupying expensive land, and permit the city to develop compactly without physical barriers. Motorways often depress nearby residential land values, but proximity to a rapid transit station often triggers commercial and residential growth, with large transit oriented development office and housing blocks being constructed.[51][53] Also, an efficient transit system can decrease the economic welfare loss caused by the increase of population density in a metropolis.[54]

Rapid transit systems have high fixed costs. Most systems are publicly owned, by either local governments, transit authorities or national governments. Capital investments are often partially or completely financed by taxation, rather than by passenger fares, but must often compete with funding for roads. The transit systems may be operated by the owner or by a private company through a public service obligation. The owners of the systems often also own the connecting bus or rail systems, or are members of the local transport association, allowing for free transfers between modes. Almost all transit systems operate at a deficit, requiring fare revenue, advertising and subsidies to cover costs.

The farebox recovery ratio, a ratio of ticket income to operating costs, is often used to assess operational profitability, with some systems including Hong Kong's MTR Corporation,[55] and Taipei[56] achieving recovery ratios of well over 100%. This ignores both heavy capital costs incurred in building the system, which are often subsidized with soft loans[57] and whose servicing is excluded from calculations of profitability, as well as ancillary revenue such as income from real estate portfolios.[55] Some metros, including Hong Kong, are even financed by the sale of land whose value has been increased by the building of the system,[36] a process known as value capture.

Environmental impacts

The Delhi Metro has won awards for environmentally friendly practices from organisations including the United Nations,[58] RINA,[59] and the International Organization for Standardization,[59] becoming the second metro in the world, after the New York City Subway, to be ISO 14001 certified for environmentally friendly construction.[60] It is also the first railway project in the world to earn carbon credits after being registered with the United Nations under the Clean Development Mechanism,[61] and has so far earned more than 400,000 carbon credits by saving energy through the use of regenerative braking systems on its trains.[62] In order to reduce its dependence on non-renewable sources of energy, Delhi Metro Rail Corporation is looking forward to harness solar energy and install solar panels in some of its metro stations.[63]


See also

Transport portal
Trains portal
Buses portal


  • Bobrick, Benson (1981). Labyrinths of Iron: a[n] History of the World's Subways. New York: Newsweek Books. ISBN 0-88225-299-2.

External links

  • (Jane's) Urban Transit Systems
  • urban rail

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