If you see someone else getting harassed, the only way you should intervene is by engaging with the person who is being harassed, not the harasser. Pretend like they don’t exist. Also call the police, metro or otherwise. Metro police are specific to the train system and will be closer to help.

There are always forced social interactions available to graduate students, go to them if you want to make friends! Worst case scenario you have awkward interactions while eating and drinking for free.