It's worth noting that the railroad which put Oelwein on the map was not the C&NW, but the Chicago Great Western, which was absorbed by the North Western in the spring of 1968.
The Great Western ran to a lot of places that counted, but almost all of them were served by paralell competitors. Under President William Deramus, who ran the CGW during the "lean-and -mean" 1950's, freight service was contracted down to one often-very-long freight in each direction daily along most CGW lines.
Oelwein was the hub where most of those lines met, but it lost almost all of its usefulness once C&NW offered a better alternative in most cases.
Here are a couple of relevant links: