Thank for all of the insight guys!! Some additional thoughts and explanations.
"The problem with your requirements is you can't have everything - high quality, RTR, accurate AND 1955 is difficult because the preponderance of accurate RTR cars for the roads on your list are skewed towards later prototypes, resulting in a mix or cars that looks more like the early 1960s than 1955."
I have worried about this and this would be problematic for me. The Park City Branch was a mining branch and most of the loads out were in GS gons of the later type similar to the Intermountain/Red caboose offering. I just don't think there is a lot of outbound box traffic that I can see. The mining and the town were pretty much in decline in the 1950's and 1960's as the mining traffic shifted to a town outside of Park City.
I am modeling fall so I will be able to use the UPHS stock car. There was still a cattle pen in Park City and other along the line. The area shipped a lot of sheep.
Inbound loads would consist of mining machinery, lumber for the lumber dealer, and lumber to support mining operations. There might be a boxcar of LCL for along the branch. We discussed reefer traffic in the past and determined that it was most likely being trucked in at this point. There was an oil dealer in town and that was still being delivered via rail. I think the rest wa probably being trucked in at this point.
The pictures I have seen of the line in the late 1940s and early 1950's show trains with a box or two, a flat or two, oil tank, general gondola, and then strings of GS gons.
Since it won't be a string of boxcars I am hoping the lack of mix/match old/new boxcar fleet will not be completely obvious.
So I am thinking my fall 1955 train looks like this....GP9, Box, Flat, 4 stock, oil, string of gons, and a caboose.
Car wise and traffic wise it's kind of a boring choice for a layout I guess but the structures and scenery are very appealing to me and no one else is doing it. Plus I think with what's available I can just recreate how the branchline was. Boring and repetitive but it will be accurate and I have always wanted to create a slice of time somewhere.