An approximate timeline of the last years of Big Boy operations:
1956 Last year of regular Big Boy operations.
November 1956 through April 1957 Final major overhauls (Classified Repairs) of all of the Big Boys. Major personnel layoffs in the Cheyenne shops.
1957 Big Boy operations continue through the summer and fall traffic surges. However, as mechanical problems develop or major inspections are due, the Big Boys are placed in storage. For example, 4006 is placed in final storage in September and 4005 and 4018 make their final trips in October.
1958 Operations of the Big Boys continue through the summer and fall. However, by October 1958, 4004 and 4018 have run their last trips. Other Big Boys follow them into the dead lines.
November 30, 1958 A system status report shows all Big Boys in storage.
July 6, 1959 4014 is the first Big Boy placed back in service for the final season.
July 7, 1959 4011, 4015, 4017 and 4023 are returned to steam.
July 8, 1959 4019 is the last Big Boy returned to steam in 1959.
July 20, 1959 4017, 4019 and 4023 make their final runs and are placed in storage.
July 21, 1959 4011, 4014 and 4015 make their final runs and are placed in storage.
July 1959 through August 1961 All of the Big Boys are retained in storage in case a sudden traffic surge requires their use. However, the diesels are able to handle the traffic.
September 1961 Scrapping of the Big Boys begins with 4015 being the first engine to be cut up.