After the failure of the Union Pacific-Rock Island merger, UP sat on the sidelines as other railroads merged over the next decade and a half. Then, in 1980, the company decided to merge–really, take over–the Missouri Pacific and Western Pacific railroads. This publication promotes that merger, which required a two-year approval process from the federal government.
Missouri Pacific was a good fit with Union Pacific. Western Pacific would also have been a good fit except for the fact that Union Pacific later took over the merged Southern Pacific-Rio Grande railroads, thus giving UP the only two lines between Salt Lake City and Oakland to the west and Denver to the east. In retrospect, Western Pacific and Rio Grande should have gone to BNSF.