IIRC - These are actually the THIRD generation of slugs used on this line.
First generation was SP's TEBU (U25B cores). SP also had four sets of yard slug - MP15AC sets and I believe at least a couple were assigned to Eugene. They had tractive effort, but not enough horsepower to create enough speed to keep up with the work. I did see some in Grand Junction, but not in use. http://espee.railfan.net/nonindex/yardslug_photos/1010a_sp-slug-jim_hanrahan.jpg
Second generation was CORP units. I recall seeing a set of these out of Medford years ago? Transferred out as their lack of dynamic brakes made them less than desirable for road service on the numerous grades where they had the extra tractive effort, but not the corresponding braking effort. I think they ended up on a line in the east that has heavy switching/transfer work they were more suited for.
Third generation, former NS built slugs: