The function d(t) represents the distance traveled by the train after t hours. d(5) is the total distance traveled by the train after 5 hours, which means 6:00 pm. The d(2) is the total distance traveled by the train after two hours, 3:00 pm. The expression d(5) - d(2) represents the total distance traveled by the train from 3:00 pm to 6:00 pm or for traveling at a constant speed for 3 hours.