I would be really interested in learning more about these very large and powerful locomotives, expecially with regard to their service: for example, I know they hauled the Portland Rose, but, apart from it, what else did they pull, both passenger or freight? Which line did they cover? How many years did they spend painted in the "two tone gray with yellow striping and lettering" livery?
The "Utah Rails" website is a great source for any and all Union Pacific research information. I have included two links pertaining to the Two-Tone Gray Steam Passenger locomotive paint scheme. The first talks about all types of steam locomotives that received this paint treatment:
The second link talks only about the Challengers:
I also managed to find a series of photos of Challenger 3977 which is on display in North Platte, NE. This photo, taken in 2002, shows 3977 in the two-tone gray passenger paint scheme: