This interactive demonstration compares how people on the train versus people on the platform view the two flashes of light emitted from the bulb. Use the slider to pick the speed of the train. Hit start and compare how the flashes of light travel from both perspectives. Notice that platform observers say the flash traveling towards the back end (left side) reaches before the flash traveling towards the front end (right side). When the train is moving slowly, the time difference between the two flashes is hardly perceptible. But when the train is moving fast, the time difference is much more pronounced.