Mo. Rev. Stat. § 388.450
Bridges may be built by railroad companies owning lines of road in the state of Missouri on the lines of their routes, across rivers and other waterways, the navigable portions of which lie wholly within the limits of the state of Missouri; provided, the location and plans thereof are submitted to and approved by the chief engineers and the Secretary of the Army of the United States, and filed in the office of the secretary of state of the state of Missouri, before construction is commenced; and provided further, that when plans for any such bridge or other structures have been approved by the chief of engineers and by the Secretary of the Army, it shall not be lawful to deviate from such plans either before or after completion of the structure unless the modification of said plans has previously been submitted to and received the approval of the chief engineer and of the Secretary of the Army.
§ 388.450, RSMo