It often happens to me, that I met some friend in the bus/tram/subway etc., but my friend is not alone, but with his/her friend (whom I don't know at all).
We are talking about something, my friend knows both of us, so there is no problem. Then my friend gets off the vehicle and only this "stranger" and I remain. If we stop talking, it is awkward. But we don't know anything about each other (sometimes not even the name). We probably won't see each other ever again, and we would probably enjoy reading a book or playing with our smartphones much more than talking to each other.
What to do in such situation? What to talk about?
(I am from Middle Europe country. A city with good public transportation, three subway routes, a lot of buses and trams.)