From a link in the article: Objections were raised at the time about the potential disruption of pre-existing patterns of travel of both passengers and merchan

From a link in the article: Objections were raised at the time about the potential disruption of pre-existing patterns of travel of both passengers and merchandise in part over Canadian rail lines. To address those objections, a proviso was added to the restated coastwise law – then the “first proviso” – which stated that the coastwise restriction did not apply to transportation “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 “excluding Alaska.” In 1935 this proviso was interpreted by the U.S. Supreme Court which wrote that “its evident purpose was to avoid disturbance of established routes, recognized by the Interstate Commerce Commission as in the public interest, between Northwestern and Eastern states through the lake ports.” End quote. The TLDR seems to be that forcing it for rail would have caused too many disturbances to how the rail was operating at the time.