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 or carriers for the pooling of freights of different and competing railroads, or to divide between them the aggregate net proceeds of the earnings of such railroads, or any portion thereof; and in case of an agreement for the pooling of freights as aforesaid, each day of its continuance shall be deemed a separate offense.