UP 3985 was part of the 4664-4 group of Challengers built in 1943. Although this group consisted of 31 locomotives, only 25 went to the Union Pacific. The other six were leased to the Denver and Rio Grande Western Railroad after completion, where they operated until the Rio Grande sold them to the Clinchfield Railroad in 1947. While the Union Pacific Challengers operated primarily in freight service, a few were assigned to passenger trains.

UP 3985 operated in its last "revenue" train service in 1957. The locomotive was officially retired in 1962 and was stored in the Union Pacific's roundhouse with Big Boy No. 4023 at Cheyenne, Wyoming. In 1975, it was brought out of storage and placed on an outdoor display beside the Cheyenne depot