By late 1954, Union Pacific was unhappy with how long-time partner Chicago & North Western handled UP passenger trains between Omaha and Chicago. Work to find a new route began, and on September 20, 1955, Milwaukee Road president John P. Kiley announced that they would handle UP's trains, including the flagship City of Los Angeles, starting October 30.
UP required cars in the Cities pool wear Armour Yellow and Harbor Mist Gray. Milwaukee planned to paint nearly 40 cars for the five trains, but quickly found the UP scheme less expensive to apply and more durable than its own. Starting in 1956, all non-commuter passenger cars and locos began receiving the new colors, which would remain the standard until Amtrak took over in 1971.
