Security camera footage broadcast by local media showed the carriages plunging towards the ground in the south of the city.
"So far we have 15 people who unfortunately lost their lives," Mexico City Mayor Claudia Sheinbaum told reporters.
The city's civil defense department said on Twitter that around 70 others were injured in the dramatic accident.
Emergency services were seen working to rescue victims from the carriages at the Olivos station.
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