Check out this interactive demonstration, which shows how light emitted from the bulb travels according to those on the train. Using the slider, pick the speed of the train. Hit start and notice what happens to the two flashes of light. Reset the animation, change the speed, and hit start again. Does the motion of the light change? No. You’ll see that the speed of the train does not matter. Because its motion is constant, people on the train can legitimately claim that they are at rest while the railroad track is passing them by. Therefore, from the train perspective, the two flashes always reach the front and back ends of the train at the same time.
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