Mexico City Mayor Claudia Sheinbaum says a train had gone into reverse for reasons that are being investigated, and subsequently crashed into another stationary train at the central Tacubaya station.

"There is going to be an investigation ... we want to know exactly why this situation happened," Sheinbaum said on Wednesday.

She added that the black boxes would be inspected and investigations would be carried out with the support of international experts.

Footage broadcast on television showed one of the trains on top of the other as police and passengers on the platform tried to help a man injured by broken glass through a jammed train door.

Most of those injured received medical attention at the scene, but 16 people had to be hospitalised, officials said.

Three subway stations were closed due to the accident.

Australian Associated Press