The Santa Fe didn't have any USRA Mikados, but the UP had 20 of them, and another 20 were allocated to UP subsidiary Oregon Short Line. A good reference guide for allocations of all types of USRA locomotives can be found on steamlocomotive.com:
http://www.steamlocomotive.com/misc/usra.shtml. The Aster model represents a USRA light Mikado. Here are a few pics of UP USRA Mikes in action:
UP 2538 - 1937 photo with original smokestack and "Union Pacific" tender lettering
UP 2484 - 1935 photo showing earlier lettering scheme with number on the tender and "Union Pacific" spelled out on the cab side
UP 2491 - 1947 photo showing the large smokestacks the UP fitted to these engines in the 1940's.
These photos all came from the Denver Public Library Western History Collection, you can search their image database here:
http://history.denverlibrary.org/images/index.html