The incident happened at Porta Garibaldi station at around 6.30am on Wednesday morning, when staff from Milan's transport association first noticed the woman walking along the platform of the green line.
They then saw her climb down from the platform and start to walk along the tunnel, seemingly oblivious to the fact that a speeding train could pass through at any moment.
Station staff immediately contacted the police, who intervened to get the woman off the tracks while the trains were temporarily stopped, Milano Today reported.
The woman, who was arrested, told police she did not need any help and had not been planning to kill herself on the tracks.
She explained that she had simply wandered along the tunnel “to visit the ancient underground city”.
The underground train service was interrupted for about ten minutes, and the woman, who was born in Milan but now lives in Naples, faces charges for causing disruption to public services.