Mayor Claudia Sheinbaum said on her Twitter account that the accident happened on Line 3 of the capital's Metro system, between the Potrero and La Raza stations.
Sheinbaum said one person was killed and 16 were taken to hospitals for injuries. But officials later raised the number of injured to 41. Four people who were trapped in the trains were rescued.
Dozens of police and soldiers swarmed into the nearby subway stations, while ambulances and rescue teams arrived to treat the injured.
Edgar Montiel was a passenger in the next-to-last car, when the rearmost car was smashed up in the crash.
"It sounded very strong. I just closed my eyes when I felt the sheets of the car bend and throw me," Montiel told The Associated Press.
He said he remained on the floor of the car with several passengers amid screams and cries asked for help.
"The power went out to the subway and a lot of smoke began to come out that was suffocating us. We could not breathe well," he said.
Montiel, who had injuries to his left arm and leg, said the occupants of his car had to wait about 30 minutes until paramedics arrived to tend to the injured and help everyone exit the car.
President Andrés Manuel López Obrador said on his Twitter account that the Mexican government was supporting the city officials dealing with accident.
In May 2021, an elevated section of the subway system collapsed, causing 26 deaths and injuring nearly 100 people. An investigation blamed the structural failure on deficiencies in the construction process, and 10 former officials were charged with homicide, injury and damage to property.
The Mexico City subway system has 226.5 kilometres of track and 195 stations. It serves an average of 4.6 million passengers every day.
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