There has been a lot of good information provided by others, but to understand it better, try to get a copy of the Hiawatha Story by Scribbins as mentioned above. If you live in the Midwest, most libraries have a copy of it. It is the "bible" for info on them.
A couple of additional comments, regarding the Skytop Cars. There were actually two different types.
The Creek Series Cars were for the Olympian Hiawatha. GGD has made the Creek Car in their set, (way out of my price range).
The Rapids Series Cars were for the Twin Cities Hiawatha. K-Line along with MTH and Lionel all have made versions of the Creek Cars. I like the K-lines best, they were offered in all three lengths (15,18,21). Like most K-Line cars, the longer the car, the higher the price. A couple of pictures of my 15" car. I think it is awesome! But I like Mainline Steam, that's why I chose that name for the Forum. I wanted to pull it with Steam. So my last point.
Page 107 of the fore mentioned book, shows an F-7 (not EMD but Milw 4-6-4 steam) pulling the consist in June 1950. All of these were retired by Aug 1951.
BTW my photo on the forum is standing next to Milw 265 at the Illinois Railway Museum.