In the Midwest, just two lines were bust, RI, and MILW. This is in constrast to all of the lines in the east, which were bust. The east needed outside intervention. Rock Island lines were snatched up by other companies, like the Spine Line to C&NW. Other marginal lines were just abandoned.
The strong lines took over the best parts of the weak ones. SOO Line took over Milwaukee, and parts of the RI were taken over, like my example of the Spine Line. There were enough strong carriers in the Midwest market left, like BN, SOO/CP, Northwestern, and ICG.