46 U.S.C. § 58108

(a) Prohibition .— A carrier may not charge, collect, or receive for transportation subject to subtitle IV of title 49 of persons or property, under any joint rate, fare, or charge, or under any export, import, or other proportional rate, fare, or charge, that is based in whole or in part on the fact that the persons or property affected are to be transported to, or have been transported from, a port in a territory or possession of the United States or in a foreign country, by a carrier by water in foreign commerce, any lower rate, fare, or charge than the carrier charges, collects, or receives for the transportation of persons or similar property for the same distance, in the same direction, and over the same route, in commerce wholly within the United States, unless the vessel used for the transportation is or was at the time of the transportation documented under the laws of the United States. (b) Suspension of Prohibition .— Whenever the Secretary of Transportation believes that adequate shipping facilities to or from any port in a territory or possession of the United States or a foreign country are not being provided by vessels documented under the laws of the United States, the Secretary shall certify this fact to the Surface Transportation Board. On receiving the certification, the Board may by order suspend the operation of subsection (a) with respect to the rates, fares, and charges for the transportation by rail of persons and property transported from or to be transported to those ports, for such time and under such terms and conditions as the Secretary may specify in the order or in any supplemental order. (c) Termination of Suspension .— Whenever the Secretary believes that adequate shipping facilities are being provided to those ports by vessels documented under the laws of the United States, and certifies that fact to the Board, the Board may order the termination of the suspension.