Concerning Milwaukee Road & Electric vs. Diesel power, there were several points to consider:
1) The Little Joes were designed to operate on the Siberian Railway @ 3,300 volts/ 4,000 amps; Milwaukee's electric system was 3,000v/3,000a. Operating 2 Little Joes at FULL power could & would overload the substations & trip the breakers. The substations on the Rocky Mountain Division were beefed up to better handle the Joes; the Coast Division never was.
2) The diesels used by Milwaukee at the time had relatively high transition speeds; in the mountains they could not get going fast enough to make transition by themselves; a Joe was added to " keep the diesels wound up."
3) It was the Joes that were modified to operate the trailing diesels, through the invention of the " Wilie Controller", named for Milwaukee Electrical Engineer Lawrence Wilie. This device linked the electric controller to a separate diesel control box with a simple pin. The diesels did not require any modifications.
4) The GE Boxcabs did not draw as many volts/amps & had lower speed continuous ratings than the Joes. This led to separation of the 2 types in service, & also allowed the Boxcabs to be used as midtrain helpers. This fact also led the Milwaukee to concentrate use of the Little Joes on the somewhat flatter Rocky Mountain Division, and the remaining Boxcabs on the steeper Coast Division.
The above information was obtained from " The Milwaukee Electrics", by Noel Holley.