Answer: The displacement is the difference in position (between the final position and the initial), so train's displacement will be the initial position subtracted of the position when the train has traveled for 4 hours.
If we define the original position as 0, then the total displacement is, if the train has a velocity of 80km/h, then in 4 hours the train travels 80*4 km= 320 km to the south. Because the train travels in one direction and never turns, the total displacement is 320km
