Infrastructure

The technology used for public, mass rapid transit has undergone significant changes in the years since the Metropolitan Railway opened publicly in London in 1863.

High capacity Monorails with larger and longer trains can be classified as rapid transit systems.citation needed Such monorail systems recently started operating in Chongqing and São Paulo. 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.

Trainsedit

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 smaller and have tunnels that restrict the size and sometimes the shape of the train compartments. One example is the London Underground which has acquired the informal term "tube train" due to its cylindrical cabin shape.

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.

Tracksedit

Most rapid transit systems use conventional standard gauge railway track. Since tracks in subway tunnels are not exposed to rain, snow, or other forms of precipitation, they are often fixed directly to the floor rather than resting on ballast, such as normal railway tracks.

An alternate technology, using rubber tires on narrow concrete or steel roll ways, was pioneered on certain lines of the Paris Métro, 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.

Some cities with steep hills incorporate mountain railway technologies in their metros. One of the lines of the Lyon Metro includes a section of rack (cog) railway, while the Carmelit, in Haifa, is an underground funicular.

For elevated lines, another alternative is the monorail, which can be built either as straddle-beam monorails or as a suspended monorail. While monorails have never gained wide acceptance outside Japan, there are some such as Chongqing Rail Transit's monorail lines which are widely used in a rapid transit setting.

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 and limiting above-ground use on the Sapporo Municipal Subway but not rubber-tired systems in other cities.

Motive poweredit

Although initially the trains of what is now the London Underground were drawn by steam engines, virtually all metro trains, both now and historically, use electric power and are built to run as multiple units. Power for the trains, referred to as traction power, usually takes 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 overhead wires. The use of overhead wires allows higher power supply voltages to be used. Although overhead wires are more likely to be used on metro systems without many tunnels, an example of which is the Shanghai Metro, overhead wires are employed on some systems that are predominantly underground, as in Barcelona, Fukuoka, Madrid, and Shijiazhuang. Both overhead wire and third-rail systems usually use the running rails as the return conductor, but 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 Boston.

Tunnelsedit

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 Kyiv 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 metres (249 ft), opened in 2017)
• Park Pobedy station in Moscow (~80 metres (260 ft), opened 2005, built under a hill)
• Puhung station in Pyongyang Metro, North Korea (which doubles as a nuclear shelter)
• Washington Park MAX Light Rail station in Portland, Oregon (built under a hill), 260 feet (80 m)

One 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.

The 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 other stations in former Soviet Union nations.

Elevated railwaysedit

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, the Miami Metrorail, and the Bangkok Skytrain.

Stationsedit

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 and elevated stations are integrated into vast underground or 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.

The deepest station in the world is Arsenalna station in Kyiv, Ukraine (105.5 m).

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 Kyiv are widely regarded as some of the most beautiful in the world. 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. 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.