I can't perform in the subway

Sometimes when I leave class and go to get on the subway I find one of the students that just left my class is waiting for the same train as me. It's not my favorite situation. I don't like trying to force a friendly conversation amidst train noise. I'm not good at faking smiles. The only time I can get into performance/teacher mode is in a classroom in front of at least five people. I just can't perform in the subway. I wonder if I would feel worse about it if my students tried to avoid me at the station. Depends on the student, I guess. Depends on whether they tried to talk about the mountain climbing gear they were about to buy or if they had something interesting on their mind.