Photo: Boxcars At Algonquin Tannery, Sault Sainte Marie, Michigan (Circa 1908)
A photo from the Library of Congress:
The TIFF version shows a lot of detail for the four boxcars pictured.
Below are some comments on the tanning process from the Early Rails Group, edited for clarity.
“The area where the photos were taken is a large industrial area and hide tanning probably would have been there. Hemlock bark was needed for tanning and Wisconsin north and east had a native area for hemlock so it makes sense that the railroads would be a likely hauler of it.”
“Notice how the open-air stacks are built with peaks to shed rain. They would sometimes or usually have a temporary “roof” of boards.”
“…the caption for the photo notes that the location is Sault Sainte Marie…and identifies the buildings mentioned as the "Algonquin Tannery" served by the Soo Line, as well as the Duluth, South Shore & Atlantic, so yes, the material stacked in the sheds was bark used in the tanning process, and it probably was hemlock.
From an earlier response to the same photo:
“The open-sided sheds contain tree bark. The tanning process derives its name from the fact that tannin (from bark) is used, hence the large supply of the stuff. The whole process passes through leather hide through several chemicals to soften it, get rid of hair and tissue and finally render the leather usable. If you have access to the Sanborn maps, the 1906 edition of Sault Sainte Marie (Page 30) has a good view of the property with the processes indicated on each building.”