At any meeting of the stockholders of any railroad corporation formed under the laws of this state for the election of directors, managers, or trustees, the stockholders may classify the directors in three equal classes, as near as may be, one of which classes shall hold office for one year, one for two years, and one for three years until its successors are respectively elected; and at all subsequent elections, in the event such classification is made, directors shall be elected for three years to fill the places made vacant by the class whose term of office expires at that time.