I also have a question about the Relativity of Simultaneity ( S&G part 1 sec. 9 ). In Einstein's thought experiment lightning strikes the tracks at each end of a train. The embankment observer who is equidistant from the strikes sees them as simultaneous. The onboard ( moving ) train observer, moving to one strike and 'away' from the other, sees them as separate events. If we alter this so that the flashes strike the tracks at the front left and right of the train, and place an observer on a footbridge who is aligned with the observer in the train, along the trains long axis, then both observers will be equidistant from the flashes and see them as simultaneous, despite being in different frames. Einstein said events which are simultaneous in the embankment frame are not simultaneous in the train frame. Can you explain. Thanks.