Union Pacific passenger car reskins, version 1, by Daniel Gollery (Trainboi1 on RWA)

In brief: This is a full reskin of the coaches included with the Jupiter, Omaha, and UP 119 packs on Steam to represent the UP's early passenger fleet as accurately as possible.

Requirements: There are three .rwp files included in this download, which are labeled by what DLC they require. UP_Passenger_Reskins_Jup requires the Jupiter DLC on Steam. UP_Passenger_Reskins_Omaha requires the Omaha DLC. UP_Passenger_Reskins_UP119 requires the UP 119 DLC. If you don't have one of these DLCs, don't install the corresponding pack. However, it is worth noting that my Omaha/Idaho and UP 119 Sisters reskin packs both require the Jupiter cars for their quickdrives, in addition to their own respective cars.

Real history: If not much is known about CP's early passenger car roster, UP's early roster is not known at all. The best we can say is this:
UP's early cars tended to keep their factory liveries as long as possible. This means that there was a mix of ornate board-and-batten cars, sleek smooth-sided cars with flowing pinstripes, and just about every variant of lettering under the sun. Most cars did not have clearly visible numbers on the side that we can see in early photos, so number ranges are completely unknown. The livery I used on these cars is based off a Monitor-roof coach photographed on a clergy excursion around 1868. This is relevant to operations, because...
Around 1866 or early 1867, flush with investment from the Pennsylvania Railroad and riding on the back of a deal with Pullman, the UP changed car colors, likely to Pullman brown. This was far from an immediate change, as evidenced by the presence of yellow cars more than a year later, but cars ordered after this point, including every known "duckbill" clerestory car on the UP, was painted brown from new. The yellow clerestory cars in this pack are meant to be representative of UP oddities, particularly the Monitor coaches and the more unusual "Turtleback" coaches.
UP's cabooses were used fairly strictly by division. DanSSG's fantastic Cheyenne extension route should mostly see the service of Laramie Division coaches, though I am not sure exactly where the Lodge Pole and Laramie divisions met, so it may be fair to see a Lodge Pole caboose riding into Cheyenne. I honestly don't know. Promontory wouldn't see UP traffic beyond about December 1869 at the latest, but what it would see would be Utah Division cabooses. There were a lot of these; I've seen them stacked onto the front of passenger trains, which didn't happen much elsewhere on the UP. Additionally, there is a great AJ Russell photo showing UP numbers 122, 123 and 120 load-testing the repaired Devil's Gate Bridge with two cabooses and no other cars.

Historical disclaimer: Next to nothing has been known about these cars for a very long time. These liveries are as accurate as I could make them, but there are a few known issues. First, the cabooses are lacking any corner post striping because the mapping would not permit it. Second, the top of the door jams on all cars are the original orange. I couldn't find a solution to this. Third and rather more importantly, the numbering on all cars save the cabooses is purely speculatory. We don't know if it was located where I placed it, or what the number range for each type of car should be. So take everything from a grain of salt.

Installation: First, get ready for a wild ride. Then, unpack the following AP files as your DLC repository allows:
Assets\Smokebox\Jupiter\JupiterAssets.ap
Assets\Smokebox\Omaha\OmahaAssets.ap
Assets\Smokebox\UP119\UP119Assets.ap
Then, install the correspending reskin RWPs to what you have. I packed each set of reskins based on DLC requirements:
For reskins on Jupiter's archroof cars (Required for all my UP passenger quickdrives):
Install UP_Passenger_Reskins_Jup.rwp using Railworks Utilities. Then, copyCPRREmigrantCoachSquaredWindows.GeoPcDx and CPRREmigrantCoachInteriorSquaredWindows.GeoPcDx from Jupiter\RailVehicles\Passenger\Coach\Default\Coach to Default\Coach UP Yellow and Default\Coach UP Brown.

For reskins on Omaha's caboose, baggage and clerestory coach (Required for Omaha's UP quickdrives):
Install UP_Passenger_Reskins_Omaha.rwp using Utilities. Then, copy UPRRCabooseCoach.GeoPcDx and UPRRCabooseCoachInterior.GeoPcDx from Omaha\RailVehicles to Omaha\RailVehicles\Caboose. Finally, copy UPRRBaggageCar.GeoPcDx, UPRRCoachClerestory.GeoPcDx and UPRRCoachClerestoryInterior.GeoPcDx from Omaha\RailVehicles to Omaha\RailVehicles\CoachesBase and Omaha\RailVehicles\CoachesBrown.
For reskins on UP 119's caboose, baggage and clerestory coach (Required for 119's UP quickdrives):
Install UP_Passenger_Reskins_UP119.rwp using Utilities. Then, copy UPRRCabooseCoach.GeoPcDx and UPRRCabooseCoachInterior.GeoPcDx from UP119\RailVehicles to UP119\RailVehicles\Caboose. Finally, copy UPRRBaggageCar.GeoPcDx, UPRRCoachClerestory.GeoPcDx and UPRRCoachClerestoryInterior.GeoPcDx from UP119\RailVehicles to UP119\RailVehicles\CoachesBase to UP119\RailVehicles\CoachesBrown.
Once all of these cars are done, make sure to test all of the cars you installed. The Jupiter pack contains two cars: one yellow archroof coach and one brown archroof coach. Find them under "UPRR" in the scenario editor with Jupiter's content folder enabled. The Omaha and 119 packs both contain three cabooses (Lodge Pole, Laramie, and Utah divisions), two baggage cars and two coaches (both Yellow/Brown). Find them respectively under "[Omaha]" and "[UP119]" in the scenario editor with the respective folder enabled.

Credits: For the issues I take with these cars' accuracy, Smokebox's help and support in this endeavor should be underlined. His cars aren't strictly accurate to each and every UP car, but to the information available, these look and feel like UP cars. He's never failed to answer a question, and while I had some incurable issues on these models, he's gone as far as providing UV layouts to make sure I can complete everything possible.
Jim Wilke, Jon Davis, Kyle Wyatt, Andrew Brandon, Randy Hees, Wendell Huffman, Curtis Reid, and many others have worked together for years to publicize the information used in creating these skins. Their importance cannot be overstated in trying to do historically accurate Western representations of any kind. Additionally, Andrew Brandon drew up vector artwork for no fewer than ten variants of UP lettering, of which I use a meager three in this set. I also shamelessly grabbed my corner post and letterboard artwork from Curtis Reid's fantastic UP passenger cars modeled for Trainz; my only consolation is that he would chide me for wasting time if I had done anything else.