A older couple begins to quarrel. As the situation dies down, we see two young people look up from their books and notice each other. Had this happened in a movie these days, they would be texting, asking each other for their Snapchat profile. Since the film is long before the time of social media, the only thing these two strangers can really do is talk.
The whole opening dialogue on the train sets up what to expect from Before Sunrise: It is not about the content of the conversations, but the outer connection of the two leads.
Nothing else to say 