Art, all the photos I have seen of the 1952-1955 Hiawatha were pulled by 3-unit A-B-A sets of FP7A's and F7B's. I had a Milwaukee Road calendar -- from 1954 I believe -- with a painting that showed the Hiawathas meeting along the Mississippi River, and both trains had the A-B-A FP7/F7 consists. In 1952, we rode the Rock Island Twin Star Rocket to and from Minneapolis, and there was one of those FP7/F7 consists in the depot at Minneapolis. I can't say that all consists of Morning and afternoon Hiawathas had these EMD's, but there's enough photographic evidence to indicate that they were commonly operated that way at the end of the orange and maroon era. Two E7's, or two Erie-Builts, were 4,000 horsepower. Three F-units were 4,500 horsepower.
As to Hiawathas on other than the Chicago-Minneapolis route, there probably was more variety, although I have seen a photo of the Midwest Hiawatha at Sioux Falls around 1953 with an A-B-A consist of FP7's bookending an F7B.
In addition to the FP7's and Erie-Builts, Milwaukee Road also owned some boiler-equipped GP9's and used them at some point in time to pull the Olympian Hiawatha through "the gap". But I do not believe that's what you were inquiring about.
The best evidence, both textual and photographic, ought to be in The Hiawatha Story, by Jim Scribbins, available from Amazon.
Just what you needed . . . Milwaukee Road info from a Southern Californian in that era. 
