As from what I understand, what makes a thing inside a moving environment behave as if they were at rest is the fact that the velocity of the environment will be "added" to whatever new velocity is generated in the environment (i.g. : throwing a ball, jumping etc..). However in that example since the speed of light is constant the people in the train and outside the train should see the exact same thing! Can anyone clarify why this reasoning is not correct ?
EDIT: To avoid watching the video, I will try to explain the example: In a moving wagon a light beam is fired in both directions (front & back)from the middle of the wagon. The people inside the wagon seen the light beam reach both ends of the wagon at the exact same time while an outside observer would see it rear end being hit first then the front end.