IRRC that may have been a result of the 1910 "Big Burn" in Idaho - Montana. That was one reason the Milwaukee went to oil fired locomotives on the west end of the system around 1912.
However, here's the interesting thing. The Northern Pacific crossed national forest lands as well, and continued to burn coal until they dropped the last fires. The NP did have some oil burners, largely to serve Portland, Oregon, where coal ceased to be available after WW I. Although coal firing predominated on the NP, a few oil burners were used out of Spokane (the last NP 4-6-6-4 to operate was an oil burner used to tamp down the track on the Noxon Dam line change in the fall of 1957, and ran back to Spokane where she was retired). Most NP oil burners ran on the Seattle - Portland line, and branches around Puget Sound. Two of the NP's Z-8 4-6-6-4s were converted to oil firing for use on the Spokane Portland & Seattle, which burned oil, hence the availability of one for use on teh new line.
Charlie Mutschler
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Charlie Mutschler