(swiped from the company's site)

According to the company's website, they believe it was an early line in eastern Montana.

Based on my own ongoing research into MDU's predecessors, I actually believe this to be a Northern Power & Light Co. substation (which served south-central ND / north-central SD). Some poles like those to the left can still be seen in service near Steele, ND, supporting my guess, while the Montana lines in question were built in the 'flat' arrangement.