The woman and her child jumped into the track when a Kavi Subhash bound train was approaching the Netaji Bhawan station a little after 9 pm, the police said.
The woman died but the child survived with injuries and has been admitted to the city hospital, they said.
The identity of the woman and the cause of suicide was not yet known, the police said adding investigations were on.
The suicide led to disruption of Metro train services for a brief period.
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