On 3 May 2021, at 22:22 CDT (UTC−5), a girder overpass in the borough of Tláhuac carrying Line 12 of the Mexico City Metro collapsed beneath a passing train. The overpass, along with the last two railcars of the train, fell onto Avenida Tláhuac near Olivos station, resulting in 26 fatalities and 98 injuries. It was the deadliest accident in the Metro's history in nearly 50 years.
Before the line opened, it faced technical and structural issues that persisted during its operation, resulting in a partial closure of the elevated section where the accident occurred, lasting from 2014 to 2015. An earthquake in 2017 further damaged the structure; although repairs were completed within a few months, residents reported that problems persisted for years. Originally announced in 2007, Line 12 was supposed to be an underground line capable of operating rubber-tired trains due to the instability of the city's soil. However, budget and time constraints led to modifications that allowed underground and above-ground operation with steel-wheeled trains. The construction was carried out by Empresas ICA, in partnership with Alstom Mexicana and Grupo Carso. It was the first railroad construction project assigned to Carso, the conglomerate owned by businessman Carlos Slim, which built the section that later collapsed.
Claudia Sheinbaum, the head of government of the city at the time of the collapse, hired the Norwegian risk management firm Det Norske Veritas (DNV) to investigate the causes of the event. Preliminary findings linked the accident to bridge construction deficiencies, including a lack of functional studs and faulty welds, fatiguing the collapsed beam. Researchers have identified the design change as a factor in track instability and damage since the line commenced operations. Further investigations concluded that the bridge had been designed and built without adhering to quality standards, that the line's construction and design changes had been inadequately supervised, and that there were insufficient fixing and safety elements. Additionally, it was found that periodic maintenance checks, which could have detected the girder buckling, had not been conducted—a claim contested by the city government.
Carso denied any wrongdoing; however, Slim agreed with the Mexican government to repair the section at no cost. In December 2021, the city's attorney general's office filed charges against ten former officials involved in the construction and supervision of the project, including its director. As of , the surviving defendants were awaiting trial on charges of manslaughter, injury, and property damage. The bridge was rebuilt, the sections constructed by Carso were reinforced, and the line underwent general maintenance, fully reopening on 30 January 2024.
Background Metro system
The Sistema de Transporte Colectivo (STC) operates the Mexico City Metro, one of the world's busiest urban transportation systems, which served approximately 4.5 million passengers daily in 2019. It was the second-largest metro system in North America in 2021.
Deterioration within the rail system resulted in multiple accidents and incidents, including a collision between two trains at Tacubaya metro station in March 2020, a railway coupler breaking while a train was operating near Misterios metro station the following month, a fire in the Metro's downtown headquarters in January 2021 that resulted in the death of a police officer, hospitalized 30 people, temporarily shut down six subway lines for several days, and a reported fire on metro tracks in April 2021.
In December 2018, the recently appointed head of government of Mexico City, Claudia Sheinbaum, designated as the general director of the STC. Serranía had previously held the same position from 2004 to 2006 during the governorship of Andrés Manuel López Obrador, who was president of Mexico at the time of the collapse. From 2020 to April 2021, the position of STC deputy director-general of maintenance was vacant, and Serranía assumed management of that role.
Line 12 and rolling stock
Line 12 operates from south-central Mexico City to the semi-rural southeastern borough of Tláhuac, serving approximately 350,000 passengers daily in 2019. In May 2021, it was the newest line in the Metro system.
The head of government of Mexico City, Marcelo Ebrard, announced the line in August 2007, and construction commenced in September 2008. Originally, the line was planned to be primarily underground, featuring 23 stations, 20 of which were to be underground (from Mixcoac to Nopalera), the section where the collapse occurred. The original budget for the project was MX$13 billion.
The opening was initially expected to coincide with the 200th anniversary of Mexico's independence in 2010. Multiple construction delays postponed its inauguration by Ebrard and required intervention from the federal government, led by the president Felipe Calderón. The project was modified to have fewer underground stations and nine elevated stations (from Culhuacán to Zapotitlán). Additionally, the elevated section would run along a different avenue to that originally planned, .
The line was constructed by Empresas ICA in association with Alstom Mexicana and Grupo Carso, the latter owned by business magnate Carlos Slim. It was Carso's first project in railway construction, and they built the bridge between Calle 11 and Zapotitlán stations. Ebrard and Calderón inaugurated the line in October 2012 a few months before the end of their terms. The project's cost soared to MX$26 billion (US$1.29 billion as of 2021).
The line runs trains, a steel-wheeled model manufactured by Construcciones y Auxiliar de Ferrocarriles (CAF). Francisco Bojórquez, the then-general director of the Metro system, advocated for rubber tires, considering them a safer option. However, an internal labor commission disagreed, supporting the more cost-effective steel wheels. Bojórquez explained that CAF trains have European specifications, but were promised to be ready one year before the proposed Bombardier trains, which followed American specifications.
Line 12 was designed similarly to Line A, the other line in the system that uses steel-wheeled trains, with considerations based on the specifications of its Bombardier trains, which are wide and weigh per railcar. In contrast, the trains are wider and heavier, measuring in width and weighing per railcar. The change was performed without considering the track requirements or reviewing the structural calculation of an elevated viaduct.
Line 12 issues
Line 12 encountered issues with trains on elevated sections during its construction. Before pre-operational testing began, topographic monitoring detected subsidence on several columns. Personnel recorded vibrations on the tracks in the Tláhuac area. Passenger testing commenced in June 2012; the Metro system noticed damage to approximately 10,000 ties due to vibrations in the following months. Operations necessitated speeds as low as on some segments to prevent damage to the tracks. A congressional report indicated that coupling unprotected rails with substandard wheels could lead to train derailments, and that Line 12 was officially certified less than an hour prior to its inauguration, despite thousands of unfinished tasks.
In March 2014, the city government closed the elevated section for 20 months for repairs due to technical and structural faults, and established a special board to investigate the errors that led to it. Thirty-three officials and former officials, including Enrique Horcasitas, the director of the Line 12 project, faced sanctions that included disqualifications from public work, fines, or both.
The administration of Miguel Ángel Mancera, Ebrard's successor, hired SYSTRA in 2014 to conduct an independent investigation and provide a report. After reviewing over 2,900 documents and testing the tracks, the team concluded that errors occurred during the "planning, design, construction, and operation" of the line. In a non-public report published by SinEmbargo.mx after the collapse, SYSTRA had noted that the wear and tear was unusual for a project that was only two years old, which they attributed to "mediocre quality" work. The group concluded that the Metro system would need to address the root causes of the wear to avoid future problems, as repairs would only serve as temporary fixes. They notified government authorities that a permanent resolution to the issues was infeasible and recommended developing a maintenance manual customized to the line's requirements for continuous operation.
The Superior Auditor of the Federation (; ASF) identified in 2015 twelve irregularities in the construction process, including the incompatibility between the train wheels and the rails, which could lead to track instability, while noting that train operations were within accepted safety limits. According to Serranía, the French company Tricaud Société Organisée (TSO) was contracted in 2016 to maintain the daily operating conditions of the line's fixed installations and reported no concerns before the collapse.
The 2017 Puebla earthquake damaged the line's tracks, leading to the temporary closure of six stations, from Tezonco metro station to the eastern terminal. The Associated Press reported that experts advised closing the line for a comprehensive inspection instead of merely reinforcing it. The ASF noted damage to the section that eventually collapsed. Following the earthquake, residents reported visible structural cracks in the structure, prompting transport authorities to address these complaints. In January 2018, they also repaired a column between Olivos and Nopalera stations that had cracks in its base.
In 2019, residents provided photographs and videos to the authorities, expressing concerns that the bridge was buckling and that the structure was moving abnormally as trains passed. By the end of that year, the engineering firm Ingeniería, Servicios y Sistemas Aplicados (ISSA) conducted a study on the structural and geotechnical behavior of the overpass, concluding that there was no risk to the line's operation.
Collapse
On 3 May 2021 at 22:22 CDT (UTC−5), an eastbound train was traveling on the track between Olivos and Tezonco stations in Tláhuac. Approximately before reaching Olivos station, the section of track between columns 12 and 13 collapsed due to a failure of the girder, causing the rear two train cars to derail. The fallen railcars formed a , with one car coming to rest on the ground and the other left suspended. Debris from the collapse struck a vehicle traveling along Avenida Tláhuac, resulting in the death of the driver and injuries to his wife. The overpass was approximately above ground level, situated over a concrete median strip, which minimized casualties among motorists and pedestrians below.
people died in the collapse, including fifteen reported deceased by midnight on 4 May, while others were hospitalized. It marked the deadliest event for the Metro since 1975, when a collision between two trains resulted in the deaths of passengers at Viaducto metro station. It was the third fatal accident within 14 months. Following the collapse, individuals were reported missing; by 6 May, the number was reduced to five. According to the city government, it is likely that none of these individuals were on board the train, as they had been missing for several days prior to the accident.