Originally Posted by Robin
The way that I heard of this thought experiment is that Tom is standing next to a railway track and he is exactly half way between two light sources A and B which are also next to the railway track. Dick is on a train that is moving from light source A to light source B.
At the instant when Dick is directly adjacent to Tom, both light sources flash simultaneously (in Tom's reference frame). Since Tom is exactly the same distance from both light sources, he will see the light from both sources at the same instant and will conclude that both light sources flashed simultaneously.
However, because Dick is moving towards light source B, he will see B's flash before he see A flash. Since the speed of light is the same in both reference frames, Dick's (equally valid) conclusion is that light source B flashed before
light source A.
I don't know what Harry was doing all this time.