You're referring to P&W's "Termite Train", which also often includes bulkhead flats of finished lumber and woodchip cars. I think that the loads of raw logs originate at mills on the Astoria Line north of St. Helens, because I've seen strings of these empty lumber cars parked on sidings at Columbia City and at Rainier.

Incidentally, I saw this train parked on on the Beburg siding in Beaverton yesterday (June 24) with a big cut of log cars and four units up front, led by former Amtrak SDP40F #644. It'sone of the 18 SDPs that Santa Fe got in a locomotive swap in 1984 and redesignated as SDF40-2s. It is now privately-owned by a Portland-area railfan and leased to P&W. This is the unit that BNSF painted in a "blue warbonnet" scheme for Maersk Sealand for the 2002 opening of its new L.A.-area intermodal terminal. It's still wearing this scheme.