There is a distinct lack of capacity between Seattle and Portland particularly anytime there are capacity issues over the cascades on Stampede/Stevens Pass due to accident, track work, or weather - then things get routed down to Portland and east thru the Columbia River Gorge.

Ironically, their used to be a third rail line over the cascades, with was flatter and honestly, better engineered - but it was ripped up in 1978-1982 era - that is the former Milwaukee Road route thru Snoqualamie pass