As per the fine SP in Oregon book, SP built the Eugene yard, roundhouse, backshop, and associated facilities in 1927, largely as part of an overall restructuring of Oregon operations as the railroad prepared to open the Natron Cutoff. Prior to that time crew change/division points had been established at Portland, Junction City, and Roseburg on the original mainline south.
The Eugene roundhouse was originally an 8-stall structure. An additional 17 open-air "garden" tracks spoked off the turntable. SP added four longer stalls to the roundhouse during World War 2 to accommodate the larger power then coming into use. The backshop contained three stalls, and the car shop contained five stalls.
The following link should take you to a Google satellite view of the old roundhouse site. The backshop building appears to still be in use today.
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maps.google.com]
Jeff Moore
Elko, NV