Well, you will need at least a few early E-unit diesels to pull the streamliners, and there may have been a few switchers.
In 1945, U.P. still had many of its old steamers -- keep your eyes open for Harriman engines built for other roads (Southern Pacific mostly). Brass P-1s are cheap, and with a new tender readily double for U.P. P-77 lights. S.P. S-10s and S-12s also can be modified for U.P. service.
Also, remember that in September 1945, the Fetters Northerns still were in coal. They were not switched to oil until the following year, in response to a threatened coal strike.
U.P. did not really dieselize until the early 1950s, in part because it owned its own coal mines, (so the fuel was free), and in part because the road recently had acquired numerous modern steam plants (the challengers and northerns). Also, the war made it impossible to get diesel units -- one needed virtually an act of Congress to secure those, and the man U.P. needed to get that accomplished -- Otto Jabelmann -- was killed during the war. A great loss to the company, but the ultimate benefit to steam fans everywhere.
Must have books: Emil Albrecht, Union Pacific Small Steam Power. Includes the Northerns and 2-10-2s (what is "small" on the Union Pacific); Wm. Kratville (I think) or perhaps James Ehrenberger, Smoke Across the Divide; Kratville & Ranks, Motive Power of the Union Pacific. And by joining the U.P. historical society, you can get a discount on the sociey'e "roster" series, showing most of the U.P.'s steam fleet by type.