The stretch of Interstate 5 between the Caliphonia / Orygun border and the Willamette Valley was one of the 1st stretches completed, in the late 1950's & early 1960's. Some stretches even date to the late 1940's & early 1950's as U.S. Hwy 99. As such , it was built to the lower maximum truck weights common at that time. This is why that stretch is in the midst of a multi - year rebuilding program. Most of the bridges in the Willamette Valley between Eugene and Salem have already been replaced with more modern structures designed for the triples and 105,500 Lbs GVW now allowed in Oregon. Similar efforts are underway in the Rogue River Valley through Medford and Grants Pass, as well as around Roseburg.
Also worth considering is the effect of many years of harsh winter weather on the original roadway through the Siskiyous. The right lane in both directions between Ashland and Yreka is beat to $#!t from years of big trucks running chains over it.
I do agree with those who doubt the return of rail traffic over Siskiyou, as the revenue just isn't there to sustain it. The near total collapse of the lumber industry in Oregon has taken away any hope for the lines' revival, and any remaining rail traffic can be routed Roseburg - Eugene - Klamath Falls - Roseville. The veneer traffic is a pittance, and at low rates too, as it is considered a raw material, as opposed to the finished plywood that is made from the veneer.