This link will take you to a comprehensive history of UP EMD diesel switchers during this era:
http://www.utahrails.net/articles/up-sw-early.php
And for Alco, Baldwin, and F-M, see:
http://www.utahrails.net/articles/up-sw-other.php
UP used 0-6-0s in various classes. See the Kratville books for steam, and Don Strack's "Union Pacific Switchers and Slugs", $24.95 from Withers Publishing, for the complete story, rosters, and photos of diesels.
RWM
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