Hello PF. Prior to the train/embankment thought experiment, Einstein argues that in order to determine if events are simultaneous, an equidistant observer is required. The embankment observer is equidistant, and when the light from the two flashes reaches him he sees them as simultaneous. The train observer is not equidistant as he is moving relative to the flashes. Therefore whilst he sees the two flashes as separate. he has no way of knowing what originally happened, because to do so requires him to be equidistant, as stated by Einstein.