It shall be unlawful for any common carrier subject to the provisions of this chapter to enter into any contract, agreement, or combination with any other common carrier for the pooling of freight of different and competing railroads, or divide between them the aggregate or net proceeds of the earnings of such railroads, or any portion thereof without the approval of the department when determined to be in the public interest by the department; and in case of an agreement for the pooling of freights without such approval, each day of its continuance shall be a separate offense.