SP & UP Gondolas
Here is another photo from the Vintage Portland website (https://vintageportland.wordpress.com/). It shows three gondolas in an area of several lumber and shingle mills in 1921. The foreground tracks were used by trolley cars.
Click on the photo to enlarge it.
The first car to the left is SP 53366. The next car is UP followed by an SP. The numbers are not clear to me.
The location was SW Water Street, near its intersection with Columbia Street. Today this area is Waterfront Park at the foot of Columbia Street.