Folks & gentry,
One Rock Island routing that hasn't been mentioned, and good until the later 1960's, would have been using the Zephyr Rocket Minneapolis to West Liberty, IA, then transfer to RI headend train #2. This connection was sometimes shown in public timetables.
Another interesting tidbit is the fact that C&NW/ Omaha Road ran Twin Cities-Chicago via two different routes: via Milwaukee and via Madison. Even in the mid 1950's there were two trains on each route and after a while connecting service between the original 400 and the Dakota 400 that provided Twin Cities-Madison-Chicago streamliner service with a change at Wyeville.
I believe there's some misunderstanding about the size and importance of the Twin Cities back in the day. In the 1930's Minneapolis had well over 400,000 people and St. Paul over 300,000, not counting suburbs. That made it larger than most metro areas, including St. Louis, Cinncy, Pittsburgh,Buffalo, KC, Dallas, Fort Worth, Houston, etc that later mushroomed in size. The Twin Cities were the home town of GN, NP, Soo Line, CGW (which started there), M&StL , Omaha Road and several short lines, not to mention 3M, Pillsbury, General Mills, IBM (I think), Minneapolis Moline, etc, etc .It was, and largely still is, the banking, marketing and logistics center for much of the midwest and is the last big city until you reach the west coast, 1800 miles away. Large concetrations of educational and medical providers reside there as well. Lots of reasons to run trains there.