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Line 12, also known as the Golden Line from its color on the system map, is a rapid transit line of the Mexico City Metro network. It travels 25.1 km along the boroughs of Benito Juárez, Iztapalapa and Tláhuac in southwestern, central-southern and southeastern Mexico City, serving twenty stations. The line was inaugurated on 30 October 2012, going from Tláhuac to Mixcoac station. In 2016, work to expand it to Observatorio station started.

Line 12 was built by Mexican construction company Empresas ICA in association with Alstom Mexicana and Grupo Carso. It runs at grade, overground and underground levels. The interchange stations are Mixcoac (Line 7), Zapata (Line 3), Ermita (Line 2) and Atlalilco (Line 8), and when completed, Observatorio (Line 1). The line connects with other transport systems in the city, including the trolleybus and the Metrobús systems. In 2019, Line 12 had a total ridership of 134,900,367 passengers, averaging 369,590 passengers per day.

Since its planning, the line underwent several modifications in its layout and characteristics. It was originally planned as a mostly subway line that would operate with rubber-tired trains. Also, the line would not operate primarily along Tláhuac Avenue. However, due to time and budget constraints the project underwent modifications after its announcement, and it became a combined under- and overground line with steel-tired trains running elevated along Tláhuac Avenue. Subsidence was reported on several columns along the elevated section before testings with trains started. From the onset of service, problems on the line were still reported especially on the elevated part of the line. In early 2014, operations were halted on that section and they were resumed until late 2015. The elevated track later suffered the impact of a earthquake in September 2017. In May 2021, a portion of the line's overpass collapsed while a train was on it. 26 people were killed and line operations had to be suspended. Preliminary investigations have suggested that the bridge had a deficient and questionable construction.

Origins
In the 1980s, the Comisión de Vialidad y Transporte Urbano (COVITUR), an organization of the Federal District Department, presented a plan for the Mexico City Metro based on several studies and reports related to the rapid growth of the city and its demand for public transportation. On the 1985 Metro Master Plan (Spanish: Programa Maestro del Metro de 1985), COVITUR considered a metro line that would run on the southern part of Mexico City from west to east and it designated as Line 12. It would run from Santa Lucía, in Álvaro Obregón, to Canal de Garay, in Iztapalapa (near Constitución de 1917 station). It would have had seventeen stations and a length of 18.97 km and it would connect with Lines 2, 3, 4, 7 and 13, the last one another planned line.

In 1996, the Sistema de Transporte Colectivo (STC), the organization responsible for the metro, published its plan for the next 25 years, the 1995 Metro and Light Rail Master Plan (Spanish: Plan Maestro del Metro y Trenes Ligero). The document considered the new demand for public transport as well as the urban expansion towards the suburbs and the neighboring State of Mexico. Line 12 appeared again in the plan, with some changes, but the route was almost the same. The 1996 Line 12 project would start in Santa Lucía and it would end at a station called Francisco del Paso (otherwise known as Del Paso or Axomulco) in Iztacalco. The idea was that both Line 8 and Line 12 would share Francisco del Paso station as a transfer station; then, the stretch of Line 8 going from this new station (and the stations between Atlalilco and Constitución de 1917) would be transferred to Line 12, while Line 8 would have a new expansion from Francisco del Paso towards Acoxpa, in Tlalpan.

Announcement
In December 2006, the government of Mexico City announced the possible construction of a new metro line to satisfy the demand for public transportation in the south of the city, taking into consideration Line 12's original plans. In July 2007, the city announced a query among its citizens, named Consulta Verde, to choose between two routes. The line would go from Mixcoac to Iztapalapa; from there, there were two choices: Iztapalapa–Acoxpa or Iztapalapa–Tláhuac. The winning route became Iztapalapa–Tláhuac. Tláhuac, one of the easternmost Mexico City boroughs, did not have metro service by then. For the project, some of the stations for Line 11 were merged into Line 12 (from Culhuacán to Periférico Oriente—with almost all the names of the stations changed).

On 8 August 2007 the project was officially presented as "Línea 12: línea dorada, la línea del Bicentenario" (Line 12: Golden Line, the Bicentennial Line) and, thus, its chosen distinctive color was golden. The project was assigned to Proyecto Metro, which was headed by Enrique Horcasitas. The announced stations from west to east, with their original names, were: Mixcoac, Insurgentes Sur, 20 de Noviembre (later renamed to Hospital 20 de Noviembre), Zapata, Parque de los Venados, Eje Central (otherwise known as Popocatépetl), Ermita, Sur 69, Vía Láctea, Mexicaltzingo, Francisco del Paso, Ganaderos (otherwise known as Granaderos), Canal Nacional, ESIME Culhuacán, La Vírgen, Calle 11, Periférico Oriente, Tezonco (also known as San Lorenzo), Olivos (also known as Los Olivos), Francisco Villa (later renamed to Nopalera), Zapotitlán, Tlaltenco and Tláhuac. All but the last three were planned as underground stations.

Construction
Due to budget, time, and other setbacks, the line underwent several modifications. For instance, Sur 69 and Vía Láctea stations were not built due to neighborhood opposition. Ganaderos station was rejected by the National Institute of Anthropology and History because its construction would have affected a nearby archaeological site. Francisco del Paso station was canceled because its construction would have caused a six-month partial closure of Line 8; instead Atlalilco station was chosen as the interchange station and as a consequence it has the longest transfer tunnel in the system. These issues caused the route to be changed. Instead of running along Arneses Avenue (Eje 3 Oriente) and later along Canal Nacional Avenue, the route was moved instead to run along Tláhuac Avenue (Ejes 9 and 10 Sur). ESIME Culhuacán and La Vírgen stations were relocated and became San Andrés Tomatlán and Lomas Estrella stations, respectively. Between Atlalilco and San Andrés Tomatlán, Culhuacán station was added to the route.

Construction of the first leg of the line, Tláhuac–Atlalilco, started on 23 September 2008. The second stretch of the line, from Atlalilco to Mixcoac was finished on April 2012. It was built by a consortium comprised by ICA Group, Carso and Alstom.

The project suffered multiple construction delays, and its $1.3 billion cost was a 50% overrun.

Route
Mixcoac station, the provisional western terminus of the line, lies along Extremadura Avenue (Eje 7 Sur), in the neighborhood of the same name. The line heads eastward along the avenue to Insurgentes Sur station at the intersection with Insurgentes Sur Avenue. Eje 7 Sur changes its name to Félix Cuevas Avenue and the track continues to Hospital 20 de Noviembre (next to the hospital of the same name). As the line reaches Zapata station, the avenue changes its name to General Emiliano Zapata Avenue, and continues to Parque de los Venados station (next to the public park of the same name). As the line reaches División del Norte Avenue, the line turns and runs along it. Then, when the route gets to Popocatépetl Avenue (Eje 8 Sur), it turns again and arrives at Eje Central station, located below the avenue of the same name. As the line arrives at Ermita station, the avenue changes its name to Calzada Ermita Iztapalapa. It passes Mexicaltzingo station and later arrives at Atlalilco station.

The line continues until the intersection with Tláhuac Avenue (Eje 9 Sur and later Eje 10 Sur) and the track emerges to the surface to become an elevated railway. It continues along the avenue to Culhuacán, San Andrés Tomatlán, Lomas Estrella, Calle 11, Periférico Oriente, Tezonco, Olivos, Nopalera and Zapotitlán stations. When Tláhuac Avenue reaches Canal del Acalote Avenue, the track turns and descends to the street level. It arrives at Tlaltenco station, and later to the southeastern terminal of Tláhuac, along San Rafael Atlixco Avenue.

Rolling stock
The FE-10 model, by Construcciones y Auxiliar de Ferrocarriles (CAF), is a steel-wheeled train model exclusively used on Line 12. As of May 2021, 30 trains are operating in the line.

Extension
A possible extension of the line to connect it with Line 1 at Observatorio station was announced in February 2013, thus making Observatorio the new west terminus of the line. The project was approved in September 2013 by Gerardo Ruiz Esparza, Secretary of Communications and Transportation.

Construction started on 29 November 2017. The extension was expected to be completed by 2020, but works were delayed, thus, being projected to open by 2021. In September 2020, it was announced that works on Line 12 will be finished by the end of 2022 and that service will begin in 2023.

The stretch will have three underground stations: Álvaro Obregón, Valentín Campa and Observatorio, in the Álvaro Obregón borough.

Ridership
According to the data provided by the authorities, the line registered 134,900,367 entrances, averaging 369,590 commuters per day in 2019. Tláhuac is the busiest station on the line—and one of the busiest of the system—averaging 56,831 entrances that year; in contrast Tlaltenco station averaged fewer than 5,000 passengers per year and consistently ranks among the least used on the Metro network.