There's a corner in the old coaches of Milan underground from where, depending on the light conditions and railway bent you can see the people sitting in front of you reflected on the back windows facing the next coach and appear as if they were sitting in there. It's the corner where people appear and fade away in a matter of seconds. I've been noticing this curious conditions since I arrived here in year 2000. Yesterday the train stopped for a while between two stations, where the rail inclination and curvature were good enough to see a steady picture.
I've been taught that optics laws respect a principle of reciprocity. That's why I thought that if one of these two "ghosts" rose their head and looked through the windows they would have seen another ghost pointing at them his mobile and take a picture.
(Made and sent from my mobile.)