Complex sentence: The "City of Denver" was a passenger train originally operated by the Chicago and North Western (CNW) and Union Pacific (UP) railroads between Chicago, Illinois, and Denver, Colorado.
Split into simpler sentences: The "City of Denver" was a passenger train operated jointly by the Chicago and North Western (CNW) and Union Pacific (UP) railroads. The train operated on both railroads' rights of way between Chicago, Illinois, and Denver, Colorado.