I wasn't aware of a major grade between Sedro Wooley and Burlington, although going westward there is heading toward sea level. It looks to not to be over 1% at worst, over 4 to 5 miles. As to Cascade Summit between Eugene and Chemult, I don't know what the maximum grade is, but if this unit grain train comes over Kicking Horse Pass west of Banff it encounters 2.2% and serious operating issues, especially in the winter.
Anyway, it appears that the old line coming south of Sumas may not be appropriate for heavy freight traffic.