Stepping in when a stranger is being harassed can be as simple as asking them for directions so they can walk away.

Intervening does not have to mean confronting anybody. What a person in that spot usually needs is an exit and somebody standing next to them, and an ordinary question about the time or the bus gives them both. The harasser loses the audience and the one-on-one, which is most of what they were relying on.

Have one line ready so you are not inventing it under pressure. Stand so you are between them if you can do it without making it a scene, and keep talking about nothing. Afterwards, ask if they want you to wait, and get staff or a driver involved if it does not defuse.