A rapid transit, metro, subway, underground, or elevated railway system is an electric passenger railway in an urban area with high capacity and frequency, and which is grade separated from other traffic. Rapid transit systems are typically either in underground tunnels or elevated above street level. Outside urban centres, rapid transit lines sometimes run grade separated at ground level. Some systems use different types in different areas.
Service on rapid transit systems is provided on designated lines between stations using electric multiple units on rails, although some systems use rubber tires. They are typically integrated with other public transport and often operated by the same public transit authorities. Rapid transit is faster and has a higher capacity than trams or light rail, but is not as fast or as far-reaching as commuter rail. It is unchallenged in its ability to transport large numbers of people quickly over short distances with little land use. Variations of rapid transit include monorails, people movers, small-scale light metro and the commuter rail hybrid S-Bahn. Today, whether any given system is considered a true rapid transit system or not depends on its configuration and implementation.
The first rapid transit system was the London Underground, which opened in 1863. The technology quickly spread to other cities in Europe and then to the United States, where a number of elevated systems were built. Since then the largest growth has been in Asia and with driverless systems. More than 160 cities have rapid transit systems, totalling more than 8,000 kilometres (5,000 mi) of track and 7,000 stations. Twenty-five cities have systems under construction.
Metro is the most common term for underground rapid transit systems, although in English speaking countries the terms subway and underground are also used. In some cities, generally in the United States and Canada, the term subway applies to the entire system, while in others only to those parts that are actually underground.
Above-ground rapid transit systems that are above street level are known by names such as elevated trains (New York City), and 'L' or el (Chicago). More recent examples are SkyTrain (Vancouver and Bangkok). Other terms for rapid transit systems are the U-Bahn and T-bane.
Rapid transit evolved from railways during the late 19th century. The first system opened was the Metropolitan Railway (now part of the London Underground) which opened in stages starting in January 1863 and eventually connected most of the main railway termini around the city. In 1890 the City & South London Railway in London was the first electric rapid transit railway. The technology swiftly spread to other cities in Europe, and then to the United States, where a number of elevated systems were built, starting in New York in 1868; this solved the problem of exhaust fumes from the steam locomotives.
By 1940, there were 19 systems, and by 1984, there were 66. This included smaller cities like Oslo and Marseille which opened extensive systems in the 1960s. More recently the growth of new systems has been concentrated in Southeast Asia and Latin America. Western Europe and North America have instead seen a revival of the tram, with light rail systems supplementing full scale urban railways, and less focus on building rapid transit. At the same time, technological improvements have allowed new driverless lines and systems. Hybrid solutions have also evolved, such as tram-train and premetro, which have some of the features of rapid transit systems.
Rapid transit is used in metropolitan areas to transport large numbers of people at high frequency. The extent of the rapid transit system varies greatly between cities, with several transport strategies: in larger metropolitan areas the underground system 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. These trains are often more expensive and less frequent, and in some cities operate only during rush hour periods. They may or may not satisfy the criteria for a rapid transit system.
Rapid transit systems are often supplemented by other systems, either buses, trams or commuter rail. Because of the high density structure of the rapid transit, short haul trips are often more easily performed on trams or buses. Many cities have chosen to operate a tram system in the city core with the metro expanding beyond it, though many cities chose to close this system in the 1950s and 60s. Another common strategy is to use a bus or tram feeder system to transport people to the transit stops and use the transits to carry them to the city center or other bus routes. This frees the suburban bus system from the requirement to drive all the way to the city center.
Rapid transit systems have high fixed costs. Most systems are publicly owned, by either local governments, transit authorities or national governments. Investments are often financed by taxation, rather than by passengers, but must often compete with funding for roads. The 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 systems operate at a deficit, requiring fare revenue, advertising and subsidies to cover costs. Hong Kong's MTR Corporation and Delhi Metro are few profitable systems without subsidies. However, some systems retain large real estate portfolios that help yield revenue—some are even financed by the sale of land whose value has been increased by the building of the system.
Each rapid transit system consists of one or more lines— each line is a specific route with trains stopping at all or some of the line's stations. Most systems operate several routes, and distinguish them by numbering, names and colors. Some lines may share track with each other, or operate solely on their own right-of-way. Often a line running through the city centre forks into two or more branches in the suburbs, allowing a higher service frequency in the centre. This arrangement is used by many systems, such as the Copenhagen Metro. Alternatively there might be a single central terminal (often shared with the central railway station), or multiple interchange stations between lines in the city center, as for instance in the Prague Metro. The Paris Métro has a unique structure with a matrix of crisscrossing lines and very closely spaced stations in the city centre. The New York City Subway  is built to serve serveral important north-south corridors in Manhattan, but is less useful for east-west trips, a consequence of the island's geography. Some far-reaching systems, such as the Madrid Metro, the Moscow Metro, the Tokyo Subway, and the London Underground, have a loop line around the city center connecting the outward lines .
The capacity of a line is obtained by multiplying together the car capacity, train length and service frequency. Heavy rapid transit trains might have six to twelve cars, while lighter systems may use only three or four cars. Cars have a capacity of 100 to 150, 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 seating needed on longer journeys. The minimum time interval between trains is shorter for with 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 1200 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.
Rapid transit operators often have built up strong brands; in particular 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. Branding has focused on easy recognition—to allow quick identification even in the vast array of signage found in large cities—combined with the need 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.
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 the rapid transit, or also include other modes of public transport.
Transit maps can be found in the transit vehicles, on the platforms, elsewhere in stations and in printed timetables. Their primary function is to help users of the system: for instance they show the interchange stations where passengers can transfer between lines. Unlike conventional maps, transit maps are usually not geographically accurate: instead they use straight lines and fixed angles, and often a fixed distance between stations, to simplify the display of the system. Often this has the effect of compressing stations in the outer area of the system and expanding those close to the center. Timetables are 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.
Rapid transits are a public space, and may suffer from security problems: petty crimes such as pickpocketing and baggage theft, and more serious crimes such as violence. 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. Rapid transits have been subject to terrorism with many casualties.
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, and systems are built to allow evacuation of trains at many places throughout the system.
Most rapid transit trains are electric multiple units with lengths from three to beyond ten cars. Power is commonly delivered by a third rail or by overhead wires. Others use the linear motor for propulsion. Most run on conventional steel railway tracks, although some use rubber tires such as the Montreal Metro. Rubber tires allow steeper gradients and a softer ride, but have higher maintenance costs and are less energy efficient. They also lose friction when weather conditions are wet or icy, preventing above ground use of the Montréal Metro but not rubber-tired systems in other cities. 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.
Underground tunnels move traffic away from street level, 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.
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.
Elevated railways are a cheap and easy way to build an exclusive right-of-way without digging expensive tunnels or creating barriers. They 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 London's Docklands Light Railway and the Bangkok Skytrain.
People mover systems are self-contained rapid transit systems serving relatively small areas such as airports, downtown 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. Monorail technology has proved difficult to commercialize and its use has been limited. The Berlin M-Bahn was the only commercial maglev rapid transit to operate, but has been closed.
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. 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.
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. 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. 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 Moscow and St. Petersburg are widely regarded as some of the most beautiful in the world, but Stockholm has also focused on art, published an art guide and offers guided tours of stations. 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.
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 means that an underground rapid transit is built in the city center, but only a light rail 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.
Commuter rail is a heavy rail system that operates at a lower frequency than rapid transit, with higher average speeds, often only serving one station in each village and town. Some cities have opted for a hybrid solution, with two tiers of rapid transit: an urban system (such as the Paris Métro and Berlin U-Bahn) and a suburban system with lower frequency (such as their counterparts RER or S-Bahn). 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 center; sometimes commuter trains have direct transfer to the rapid transits, on the same or adjoining platforms.
More than 160 cities have built rapid transit systems, and about twenty-five have new systems under construction. The capital cost is high, with public financing 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 push down nearby residential land values, but proximity to a rapid transit station often triggers commercial and residential growth, with large office and housing blocks being constructed.
A Rapid transit (or Metro(politan), subway, underground, tube, elevated system) is a railway system. Usually it is located in a city or urban area. Trains run very often, and carry a lot of passengers. Usually it is separated from other traffic by a concept called grade separation. Grade separation is when two streams of traffic cross each other on different levels, so they do not influence each other. In England it is called Undergound, in Scotland and North America it is called Subway, in Germany and Austria U-Bahn and in the rest of the world usually Metro, for example: Barcelona Metro.