Modern services on rapid transit systems are provided on designated lines between
stations typically using
electric multiple units on
rail tracks. Some systems use guided rubber tires, magnetic levitation (maglev), 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. Some rapid transit systems have at-grade intersections between a rapid transit line and a road or between two rapid transit lines.
Metro is the most common term for underground rapid transit systems used by non-native English speakers. Rapid transit systems may be named after the medium by which passengers travel in busy
central business districts; the use of
tunnels inspires names such as subway,underground,Untergrundbahn (
U-Bahn) in German, or the Tunnelbana (T-bana) in Swedish. The use of
viaducts inspires names such as elevated (L or el), skytrain,overhead, overground or Hochbahn in German. 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.
New York City Subway is referred to simply as “the subway”, despite 40% of the system running above ground. The term “L” or “El” is not used for elevated lines in general as the lines in the system are already designated with letters and numbers. The “L” train or
L (New York City Subway service) refers specifically to the 14th Street–Canarsie Local line, and not other elevated trains.
The opening of London's steam-hauled
Metropolitan Railway in 1863 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 was the first electric-traction rapid transit railway, which was also fully underground. Prior to opening, the line was to be called the "City and South London Subway", thus introducing the term Subway into railway terminology. Both railways, alongside others, were eventually merged into
London Underground. The 1893
Liverpool Overhead Railway was designed to use electric traction from the outset.
The technology quickly spread to other cities in Europe, the United States, Argentina, and Canada, with some railways being converted from steam and others being designed to be electric from the outset.
New York City all converted or purpose-designed and built electric rail services.
Advancements in technology have allowed new automated services. Hybrid solutions have also evolved, such as
premetro, which incorporate some of the features of rapid transit systems. In response to cost, engineering considerations and topological challenges some cities have opted to construct tram systems, particularly those in Australia, where density in cities was low and
suburbs tended to
spread out. Since the 1970s, the viability of underground train systems in Australian cities, particularly
Melbourne, has been reconsidered and proposed as a solution to over-capacity. Melbourne had tunnels and stations developed in the 1970s and opened in 1980. The
first line of
Sydney Metro, was opened in 2019.
Since the 1960s, many new systems were introduced in
Latin America. In the 21st century, most new expansions and systems are located in Asia, with China becoming the world's leader in metro expansion, operating some of the largest and busiest systems while possessing almost 60 cities that are operating, constructing or planning a
rapid transit system.
Rapid transit is used in
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.
Rapid transit systems may be supplemented by other systems such as
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.
Each rapid transit system consists of one or more lines, or circuits. 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, the
Milan Metro, the
Oslo Metro, the
Istanbul Metro and the
New York City Subway.
Alternatively, there may be a single central terminal (often shared with the central railway station), or multiple interchange stations between lines in the city center, for instance in the
Prague Metro. The
London Underground and
Paris Métro 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
Koltsevaya Line and
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. The minimum time interval between trains is shorter for rapid transit than for mainline railways owing to the use of
Communications based train control: the minimum headway can reach 90 seconds, but many systems typically use 120 seconds to allow for recovery from delays. Typical capacity lines allow 1,200 people per train, giving 36,000
passengers per hour per direction. However, much higher capacities are attained in
East Asia with ranges of 75,000 to 85,000 people per hour achieved by
MTR Corporation's urban lines in Hong Kong.
topologies are determined by a large number of factors, including geographical barriers, existing or expected travel patterns, construction costs, politics, and historical constraints. A transit system is expected to serve an area of land with a set of lines, which consist of shapes summarized as "I", "L", "U", "S", and "O" shapes or loops. Geographical barriers may cause chokepoints where transit lines must converge (for example, to cross a body of water), which are potential congestion sites but also offer an opportunity for transfers between lines.
Ring lines provide good coverage, connect between the radial lines and serve tangential trips that would otherwise need to cross the typically congested core of the network. A rough grid pattern can offer a wide variety of routes while still maintaining reasonable speed and frequency of service. A study of the 15 world largest subway systems suggested a universal shape composed of a dense core with branches radiating from it.
Rapid transit operators have often built up strong
brands, often 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. 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.
Shenzhen Metro uses large LCD information displays to show the current location, upcoming stops and diagrams of the next station.
transit map is a
topological map or
schematicdiagram 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. Transit maps can be found in transit vehicles, on
platforms, elsewhere in stations, and in printed
timetables. Maps help users understand the interconnections between different 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.
Some systems assign unique
alphanumeric codes to each of their stations to help commuters identify them, which briefly encodes information about the line it is on, and its position on the line. For example, on the
Changi Airport MRT station has the alphanumeric code CG2, indicating its position as the 2nd station on the Changi Airport branch of the East West Line. Interchange stations have at least two codes, for example,
Raffles Place MRT station has two codes, NS26 and EW14, the 26th station on the North South Line and the 14th station on the East West Line.
The Seoul Metro is another example that utilizes a code for its stations. Unlike that of Singapore's MRT, it is mostly numbers. Based on the line number, for example Sinyongsan station, is coded as station 429. Being on Line 4, the first number of the station code is 4. The last 2 numbers are the station number on that line. Interchange stations can have multiple codes. Like City Hall station in Seoul which is served by Line 1 and Line 2. It has a code of 132 and 201 respectively. The Line 2 is a circle line and the first stop is City Hall, therefore, City Hall has the station code of 201. For lines without a number like Bundang line it will have an alphanumeric code. Lines without a number that are operated by KORAIL will start with the letter 'K'.
With widespread use of the
cell phones globally, 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.
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
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.
High platforms, usually over 1 meter / 3 feet, are a safety risk, as people falling onto the tracks have trouble climbing back.
Platform screen doors are used on some systems to eliminate this danger.
Some subway systems, such as the
Beijing Subway, which is ranked by Worldwide Rapid Transit Data as the "World's Safest Rapid Transit Network" in 2015, incorporates airport-style security checkpoints at every station. Rapid transit systems have been subject to
terrorism with many casualties, such as the 1995
Tokyo subway sarin gas attack and the 2005 "
7/7" terrorist bombings on the London Underground.
Some rapid transport trains have extra features such as wall sockets, cellular reception, typically using a
leaky feeder in tunnels and
DAS antennas in stations, as well as
Wi-Fi connectivity. The first metro system in the world to enable full mobile phone reception in underground stations and tunnels was Singapore's Mass Rapid Transit (MRT) system, which launched its first underground mobile phone network using
AMPS in 1989. Many metro systems, such as the Hong Kong Mass Transit Railway (MTR) and the Berlin U-Bahn, provide mobile data connections in their tunnels for various network operators.
Monorails with larger and longer trains can be classified as rapid transit systems. Such monorail systems recently started operating in
Light metro is a subclass of rapid transit that has the speed and grade separation of a "full metro" but is designed for smaller passenger numbers. It often has smaller loading gauges, lighter train cars and smaller consists of typically two to four cars. Light metros are typically used as
feeder lines into the main rapid transit system. For instance, the
Wenhu Line of the
Taipei Metro serves many relatively sparse neighbourhoods and feeds into and complements the high capacity metro lines.
Some systems have been built from scratch, others are reclaimed from former commuter rail or suburban tramway systems that have been upgraded, and often supplemented with an underground or elevated downtown section. At grade alignments with a dedicated
right-of-way are typically used only outside dense areas, since they create a physical barrier in the urban fabric that hinders the flow of people and vehicles across their path and have a larger physical footprint. 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.
Most rapid transit trains are
electric multiple units with lengths from three to over ten cars. Crew sizes have decreased throughout history, with some modern systems now running completely unstaffed trains. 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. 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.
Some urban rail lines are built to a
loading gauge as large as that of
main-line railways; others are built to a smaller one and have
tunnels that restrict the size and sometimes the shape of the train compartments. One example is most of the
London Underground, which has acquired the informal term "tube train" due to the cylindrical shape of the trains used on the
deep tube lines.
In many cities, metro networks consist of lines operating different sizes and types of vehicles. Although these sub networks are not often connected by track, in cases when it is necessary, rolling stock with a smaller
loading gauge from one sub network may be transported along other lines that use larger trains. On some networks such operations are part of normal services.
An alternate technology, using
rubber tires on narrow
concrete or steel
roll ways, was pioneered on certain lines of the
Paris Métro and
Mexico City Metro, and the first completely new system to use it was in
Montreal, Canada. On most of these networks, additional horizontal wheels are required for guidance, and a conventional track is often provided in case of
flat tires and for
switching. There are also some rubber-tired systems that use a central
guide rail, such as the
Sapporo Municipal Subway and the
NeoVal system in
Rennes, France. Advocates of this system note that it is much quieter than conventional steel-wheeled trains, and allows for greater
inclines given the increased
traction of the rubber tires. However, they 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 and limiting it on the Sapporo Municipal Subway, but not rubber-tired systems in other cities.
Although trains on very early rapid transit systems like the
Metropolitan Railway were powered using
steam engines, either via cable haulage or
steam locomotives, nowadays virtually all metro trains use
electric power and are built to run as
multiple units. Power for the trains, referred to as
traction power, is usually supplied via one of two forms: an
overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings, or a
third rail mounted at track level and contacted by a sliding "
pickup shoe". The practice of sending power through rails on the ground is mainly due to the limited overhead clearance of tunnels, which physically prevents the use of
The use of overhead wires allows higher power supply
voltages to be used. Overhead wires are more likely to be used on metro systems without many tunnels, for example, the
Shanghai Metro. Overhead wires are employed on some systems that are predominantly underground, as in
Shijiazhuang. Both overhead wire and third-rail systems usually use the running rails as the return conductor. Some systems use a separate fourth rail for this purpose. There are transit lines that make use of both rail and overhead power, with vehicles able to switch between the two such as
Blue Line in
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
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
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 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,
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 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.
A disadvantage with this, 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. It 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
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. Usually, the vertical distance between the ground level and the rail is used to represent the depth. Among the possible candidates are:
An advantage of deep tunnels is that they can dip in a basin-like profile between stations, without incurring the significant extra costs associated with digging near ground level. This technique, also referred to as putting stations "on humps", allows gravity to assist the trains as they accelerate from one station and brake at the next. It was used as early as 1890 on parts of the
City and South London Railway and has been used many times since, particularly in Montreal.
West Island line, an extension of the
MTR Island line serving western Hong Kong Island, opened in 2015, has two stations (
Sai Ying Pun and
HKU) situated over 100 metres (330 ft) below ground level, to serve passengers on the
Mid-levels. They have several entrances/exits equipped with high-speed lifts, instead of
escalators. These kinds of exits have existed in many London Underground stations and stations in former Soviet Union nations.
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
Docklands Light Railway, the
Miami Metrorail, and the
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
side platforms. 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 and elevated stations are integrated into vast
skyway networks respectively, that connect to nearby commercial buildings. In suburbs, there may be a "
park and ride" connected to the station.
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, 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.
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
Kyiv are widely regarded as some of the most beautiful in the world. Several other cities such as London,Stockholm,
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. It may be possible to profit by attracting more passengers by spending relatively small amounts on grand
lighting and a feeling of
In the early days of underground railways, at least two staff members were needed to operate each train: one or more attendants (also called "
conductor" or "guard") to operate the doors or gates, as well as a driver (also called the "
engineer" or "motorman"). The introduction of powered doors around 1920 permitted crew sizes to be reduced, and trains in many cities are now operated by
a single person. Where the operator would not be able to see the whole side of the train to tell whether the doors can be safely closed,
closed-circuit TV monitors are often provided for that purpose.
A replacement system for human drivers became available in the 1960s, with the advancement of
computerized technologies for
automatic train control and, later,
automatic train operation (ATO). ATO could start a train, accelerate to the correct speed, and stop automatically in the correct position at the
railway platform at the next station, while taking into account the information that a human driver would obtain from
cab signals. The first metro line to use this technology in its entirety was London's
Victoria line, opened in 1968.
In normal operation, a crew member sits in the driver's position at the front, but is only responsible for closing the doors at each station. By pressing two "start" buttons the train would then move automatically to the next station. This style of "semi-automatic train operation" (STO), known technically as "
Grade of Automation (GoA) 2", has become widespread, especially on newly built lines like the San Francisco Bay Area's
A variant of ATO, "driverless train operation" (DTO) or technically "GoA 3", is seen on some systems, as in London's
Docklands Light Railway, which opened in 1987. Here, a "passenger service agent" (formerly called "train captain") would ride with the passengers rather than sit at the front as a driver would, but would have the same responsibilities as a driver in a GoA 2 system. This technology could allow trains to operate completely automatically with no crew, just as most
elevators do. When the initially increasing costs for
automation began to decrease, this became a financially attractive option for employers.
At the same time, countervailing arguments stated that in an
emergency situation, a crew member on board the train would have possibly been able to prevent the emergency in the first place, drive a partially failed train to the next station, assist with an
evacuation if needed, or call for the correct
emergency services and help direct them to the location where the emergency occurred. In some cities, the same reasons are used to justify a crew of two rather than one; one person drives from the front of the train, while the other operates the doors from a position farther back, and is more conveniently able to assist passengers in the rear cars. An example of the presence of a driver purely due to union opposition is the
Scarborough RT line in Toronto.
Platform screen doors at Castle Hill Station on the
Systems that use automatic trains also commonly employ full-height
platform screen doors or half-height
automatic platform gates in order to improve safety and ensure passenger confidence, but this is not universal, as networks like
Nuremberg do not, using
infrared sensors instead to detect obstacles on the track. Conversely, some lines which retain drivers or manual train operation nevertheless use PSDs, notably London's
Jubilee Line Extension. The first network to install PSDs on an already operational system was
Hong Kong's MTR, followed by the Singapore MRT.
As for larger trains, the
Paris Métro has human drivers on most lines but runs automated trains on its newest line,
Line 14, which opened in 1998. The older
Line 1 was subsequently converted to unattended operation by 2012, and it is expected that
Line 4 will follow by 2023. The
North East MRT line in Singapore, which opened in 2003, is the world's first fully automated underground urban heavy-rail line. The MTR
Disneyland Resort line is also automated, along with trains on the
South Island line.
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.
premetro design means that an underground rapid transit system is built in the city center, 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
In some cases, such as the
London Underground and the
London Overground, suburban and rapid transit systems even run on the exact same track along some sections.
Metrorail system is an example of a hybrid of the two: in the suburbs the lines function like a commuter rail line, with longer intervals and longer distance between stations; in the downtown areas, the stations become closer together and many lines
interline with intervals dropping to typical rapid transit headways.
As of March 2018[update], 212 cities have built rapid transit systems. 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; the rapid transit system allows higher capacity with less land use, less environmental impact, and a lower cost.
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. Also, an efficient transit system can decrease the economic welfare loss caused by the increase of
population density in a metropolis.
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
subsidies to cover costs.
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, and
Taipei 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 and whose
servicing is excluded from calculations of profitability, as well as ancillary revenue such as income from
real estate portfolios. Some systems, particularly Hong Kong's, extensions are partly financed by the sale of land whose value has appreciated by the new access the extension has brought to the area, a process known as
Urban land-use planning policies are essential for the success of rapid transit systems, particularly as mass transit is not feasible in low-density communities. Transportation planners estimate that to support rapid rail services, there must be a residential housing density of twelve dwelling units per acre.
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