In a news conference, Mayor Claudia Sheinbaum said a train driver was in a serious condition following the incident at the La Raza and Potrero stations on metro line 3.
Four other people were hospitalized after they were rescued in an operation involving the Defense Ministry, Navy and the Civil Protection agency, the mayor said.
“As always, our priority is the victims and also that justice will be done,” she said.
Mexico City’s prosecutor’s office wrote on Twitter that it has started a government-authorized investigation into the collision.
The city’s subway is one of the busiest public transit systems in the world, serving a metropolitan area home to an estimated 20 million people in the densely populated capital.
The collision is the latest deadly incident to hit the Mexico City metro following the May 2021 collapse of subway Line 12 that killed 26 people and left dozens injured.
According to a 2021 report by the city’s government, construction flaws led to that collapse.
The investigation suggested that deficient welding of metal studs, which apparently were not well connected to steel beams supporting a concrete slab and the elevated train rails, was among a number of issues that contributed to the incident.
The report said missing metal studs in some sections of the structure, different kinds of concrete used for the slab and unfinished or badly welded joints were some of the other factors that caused the raised railway to buckle, sending two subway carriages plummeting to the streets below.
CNN’s Rafael Romo and Natalie Gallón contributed reporting.