The first division to be electrified was the Rocky Mountain Division from Harlowton, Montana to Avery, Idaho.  This covered a distance of and began electric operation in 1917. The electrification remained in operation until 1974, when diesel locomotives took over.  There were two main reasons for electrifying this division.  The first was to get through the Bitter Root Mountains, which are steeply graded.  The second was that the line passes through an important forest reserve of the US Government. Steam trains were a fire hazard, and thus electric trains lessened the risk.