This demonstration shows the time difference that platform observers will measure between the two light flashes reaching President Forwardland and President Backwardland. (In the train’s frame, the two light flashes reach the two presidents simultaneously.) Use the first slider to select the speed of the train, which will move the crosshairs along the curve. Use the second slider to select the length of the train, which will change the shape of the curve. Observe that the faster the train moves, and the greater the length of the train, the greater the difference an observer in the ground frame will read between the two clocks, although a significant time difference arises only when the train’s speed is extraordinarily close to the speed of light.