Infrastructure
La mayoría de los trenes de ferrocarriles metropolitanos son de unidades de tren eléctricas, con longitudes de dos a más de diez coches. La electricidad para las motorizaciones eléctricas es provista por un tercer carril "Tercer riel (alimentación)") o catenaria "Catenaria (ferrocarril)"). Otro sistema de propulsión en algunos trenes es el motor lineal.
La mayoría circulan en vías férreas de acero convencionales, aunque algunos utilizan neumáticos de goma, como el Metro de Montreal. Los neumáticos de goma posibilitan circular por pendientes empinadas, pero generan mucho ruido, tienen mayores costes de mantenimiento y son menos eficientes energéticamente. También pierden la fricción cuando las condiciones climáticas son húmedas o heladas, estando limitado su uso en superficie en el Metro de Montreal.
El tamaño de la tripulación ha disminuido a través de la historia con algunos sistemas modernos con plena operación automática del tren (ATO) (como los sistemas VAL, SAET, el sistema de AnsaldoBreda o el SelTrac) que permiten el funcionamiento del tren sin conductor. Algunas redes que utilizan este sistema son el Metro de Copenhague, el Metro de Rennes, el Docklands Light Railway de Londres, las líneas 1 "Línea 1 (Metro de París)") y 14 "Línea 14 (Metro de París)") del Metro de París, las líneas 9, 10 y 11 del Metro de Barcelona, las líneas 4 "Línea 4 (Metro de São Paulo)"), 6 "Línea 6 (Metro de São Paulo)"), 15 "Línea 15 (Metro de São Paulo)") y 17 "Línea 17 (Metro de São Paulo)") del Metro de Sao Paulo y las líneas 3 y 6 del Metro de Santiago de Chile. Otros trenes siguen teniendo conductores, aun cuando su único papel es abrir y cerrar las puertas de los trenes en las estaciones, como el Tren Urbano de San Juan y el Metro de Santiago de Chile.
En algunos sistemas metropolitanos se utiliza el esquema de medición neta, como en el caso de España, para no derrochar electricidad.
La red del Metro de Tokio se compone de 13 líneas, sin incluir la red de trenes urbanos y suburbanos (cercanías) de Japan Railways (JR), entre la que vale destacar la línea Yamanote, la vía férrea de mayor tráfico y comunica los 23 distritos especiales de la urbe.
Trains
Most rapid transit trains are electric multiple units with lengths from three to more than ten cars.[19] Crew sizes have decreased throughout history, with some modern systems now running unmanned trains.[20] Other trains continue to have drivers, although their only role in normal operation is to open and close train doors at stations. Power is commonly delivered by third rail or overhead cables. The entire London Underground network uses the fourth rail and others use the linear motor for propulsion.[21].
Some urban railway lines are built with a loading gauge as large as that of the main railway lines; others are built to be smaller and have tunnels that restrict the size and sometimes the shape of the train compartments. An example is the London Underground, which has acquired the informal term "tube train" due to the cylindrical shape of its cabin.
In some cities, suburban rail networks consist of lines that operate different sizes and types of vehicles. Although these subnetworks are not usually connected by track, where necessary, rolling stock with a smaller loading gauge from one subnetwork can be transported along other lines that use larger trains.
Sometimes pneumatic rolling is used for uneven routes or continuous slopes, such as 10 of the 12 lines of the Mexico City Metro, or some of the Paris metro.
Tracks
Most rapid transit systems use standard gauge conventional railway tracks. Since underground tunnel tracks are not exposed to rain, snow, or other forms of precipitation, they are often fixed directly to the ground rather than resting on ballast, like normal train tracks.
driving force
Although initially the trains of what is now the London Underground were pulled by steam engines, virtually all tube trains, both now and historically, use electrical power and are built to operate as multiple units. Train power, known as traction power, generally takes one of two forms: an overhead line, suspended from poles or towers along the track or from tunnel structures or roofs, or a third rail mounted at track level and in contact with a sliding cable.
The practice of sending power via rails in the ground is primarily due to the limited overhead clearance of tunnels, which physically precludes the use of overhead cables. The use of overhead cables allows higher power supply voltages to be used. Although overhead cables are more likely to be used in subway systems without many tunnels, an example of which is the Shanghai Metro, overhead cables are used in some systems that are predominantly underground, such as in Barcelona, Fukuoka, Hong Kong, Madrid and Shijiazhuang. Both overhead cable and third rail systems generally use the 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 air power, with vehicles that can switch between the two, such as the Blue Line "Blue Line (Boston Subway)") in Boston.
Tunnels
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 economical route for mass transportation. Cut and cover tunnels are built by excavating city streets, which are then rebuilt over the tunnel; Alternatively, tunnel boring machines can be used to excavate deep tunnels that lie lower in the bedrock. [28].
Construction of an underground subway is an expensive project and often takes place over several years. There are several different methods for building underground lines.
• - In a common method, known as cut and cover, 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 road rebuilt. This method often involves extensive relocation of utilities that are normally buried not far below street level, particularly electrical and telephone wiring, water and gas mains, and sewers.
• - Another common type of tunneling method is called perforated tunneling. Here, construction begins with a vertical axis from which tunnels are excavated horizontally, often with a tunnel shield, thus avoiding almost any disturbance to existing streets, buildings and public services. But problems with groundwater are more likely, and tunneling through native bedrock may require blasting.
• - Other lines were built with cut and cover and have since been equipped with air-conditioned trains.
The world's deepest subway system was built in St. Petersburg, Russia, where in the swampy area, stable ground begins more than 50 meters (160 feet) deep.
Elevated railways
Elevated railroads are a cheaper and easier way to build an exclusive right-of-way without digging costly tunnels or creating barriers. Besides street-level railways, they may also be the only other feasible alternative due to considerations such as a high water table near the city surface increasing the cost or even preventing underground railways (e.g. Miami). Elevated guides were popular at the turn of the century, but fell out of favor; They came back into fashion in the last quarter century, often in combination with ductless systems. In the Santiago de Chile metro, parts of line 2 (inaugurated in 1978); line 5 (inaugurated in 1997); line 4 (inaugurated in 2005-06); and expansion of line 5 (inaugurated in 2011), have elevated tracks.
Variations
However, not all cities in the world can count on this type of transportation; in cities with weak soil (lack of consistency) and located in seismic zones, its cost would rise by almost 300% of what it would cost to build this in another city. The case of the Seville metro, because the water table was too high in addition to the type of marsh terrain, also represented an added technical difficulty.
Although there are urban railways whose route runs totally or partially on the surface, such as that of Maracaibo, the concept of "metro" is generally associated with underground railways, a solution that was progressively adopted by cities that had not originally adopted it, due to several reasons, among which may be the superiority in the order of aesthetic and environmental quality of the underground route, as well as the lack of available land or the cost of land in large cities.[22].
When the subway circulates under the open sky, the tracks are generally placed on metal or concrete platforms elevated about four or five meters above the ground,[23] so that the subway does not interfere with street traffic. However, its noise can be annoying to neighbors, so in some cities, such as São Paulo, acoustic barriers were installed to reduce the noise caused by the movement of trains.[24] In others, such as Santiago de Chile or Paris, the trains that circulate on metro lines that run partially under the open sky are equipped with cars running on tires, which provides considerable silence and running comfort. In others, such as Prague, or line 2 of the São Paulo Metro "Line 2 (São Paulo Metro)") the journey on the surface is carried out inside elevated tubes.
Since the electrification of railways, the subway has become a means of electric transportation throughout the world. In some cases the current is conducted by catenaries "Catenaria (railway)") above the train (sometimes rigid, as in Madrid, more efficient) and, in others, there are special tracks intended for this task on the sides of the route (as is the case, for example, of the London or Santiago subways).
In recent years, metropolitan railway system operators have embarked on the construction and operation of light rail lines, which due to their peculiar construction and operation characteristics are considered independent of conventional lines.