In 1937, the Chicago, Burlington and Quincy Railroad (Burlington Route) needed backup locomotives for their streamlined diesel-hauled Zephyr passenger trains. Their solution was to streamline their Baldwin-built no. 3002 in their main Iowa shops. The locomotive was renumbered as no. 4000 and given the name Aeolus, after the mythical keeper of the winds. A second streamlined 4-6-4 was built for this purpose and numbered 4001.