Santiago: I know you aren't an enthusiast of the O1a, but Denver had a number of them assigned through 1955 and presume switching was one of or the primary tasks they performed. Those O1a's were retired after 1955. That certainly overlaps the Zephyr era, and certainly the Aristocrat's period of operation.
Then in Sept 1958, due to two catastrophic wrecks on the Colorado & Southern which destroyed about 5 diesel locomotives, the C&S asked Burlington's Chicago headquarters for help - they sent out 5 O1a's, which worked from Oct 1958 to about Feb 1959. O1a's were long lived and versatile locomotives for the Burlington. Those O1a's did not work Burlington trains but were visible switching the C&S yard adjacent to Denver Union Station. One of those O1a's has been preserved in Mendota, IL (#4978, with Elesco feed water heater. That specific locomotive was done by Sunset 3rd Rail).