I believe I can help here....
For starters... this paint scheme came about in the post war. (WWII) As for the locos that pulled them. Not just the Challengers and 484 (Northern Pacifics) pulled them. There were others that pulled them like the Mikes (mikado) 2-8-2s and even the heavy 2-10-2 on occasions and later some diesels E & F as well as the Alco units.
According to some of my readings and photos these passenger cars were built in 1921 and rebuilt @1941 to this scheme. Some people have dubbed these to be the Challenger scheme because they mostly ran on the Challenger route to the City of Los Angeles. (also known as the COLA train)
As far as getting photos and the like I can recommend the books by Lou Schmitz, Jim Ehernberger, Don Strack and R.H Kindig. There are many others. Withers Publishing and Morning Sun Books have most of what you are looking for and of coures the UPHS (Union Pacific Historical Society) See my signature below.
I would like to post pics but I do not have a scanner plus I would not like to do something like that without permission.
This website can help you alot. The writings of Don Strack. Although he doesnt mention other locos but there are evidence of other locos pulling this 2 tone gray scheme. Don tends to have only interests in the Utah history of RR but on occasions he does some books on UPRR. There other gents that offer more in depth of information at the UPHS that can help. As I said I have photos but I do not want to violate any laws or even my relations of the original owner's work.
I hope I have helped and getting you in the right direction.