Sec. 27.
That no merchandise shall be transported by water, or by land and water, on penalty of forfeiture thereof, between points in the United States, including Districts, Territories, and possessions thereof embraced within the coastwise laws, either directly or via a foreign port, or for any part of the transportation, in any other vessel than a vessel built in and documented under the laws of the United States and owned by persons who are citizens of the United States, or vessels to which the privilege of engaging in the coastwise trade is extended by sections 18 or 22 of this Act: Provided, That this section shall not apply to merchandise transported between points within the continental United States, excluding Alaska, over through routes heretofore or hereafter recognized by the Interstate Commerce Commission for which routes rate tariffs have been or shall hereafter be filed with said commission when such routes are in part over Canadian rail lines and their own or other connecting water facilities: Provided further, That this section shall not become effective upon the Yukon river until the Alaska Railroad shall be completed and the Shipping Board shall find that proper facilities will be furnished for transportation by persons citizens of the United States for properly handling the traffic.