Mo. Rev. Stat. § 388.070

Any railroad corporation heretofore or hereafter organized under the laws of this state may elect its directors for one or more years, not to exceed three years, the time of service and mode of classifications to be provided for by the bylaws of the corporation; provided, however, that there shall be an annual election for such number or proportion of directors as may be found upon dividing the entire number of directors by the number of years composing a term.