So, based on the merger history of the railroads that operated the paired track, these are the eras:
The SP / WP era: 1976 - 1982In this era, the Southern Pacific and Western Pacific were the operators of the paired track arrangement across Nevada as they had been for many years. A few of Western Pacific's locomotives are still in the silver and orange paint scheme but most are in one of the green and orange schemes.
The SP / UP era: 1983 - 1988During these years, the Western Pacific has been merged into the Union Pacific. UP's paint shops were fairly quick to assemlate most of the aquired WP locomotives as well as those aquired from other railroads such as the MKT (Missouri, Kansas, and Texas) and Rock Island.
The UP / DRGW era: 1989 - 1996
In this era the Southern Pacific Railroad has been purchased by Rio Grande Industries which also owned the Denver & Rio Grande Railroad. They merged the two railroads and kept the Southern Pacific name and the scarlet and gray paint scheme but changed the locomotive lettering to a style similar to what the Rio Grande had used.
In the Midwest, the UP has taken over the Chicago & Northwestern and the Missouri Pacific and by now UP's paint shops are only patching the UP emblem and new road numbers on the the acquired locomotives. As those locomotives find their way into the western reaches of the UP system, it means even more variety of paint schemes.
Besides using different railroad equipment that would be appropriate for each era, vehicles, structures, and other details could be exchanged to show the layout in different periods of time. At this point I began to expand my locomotive roster to cover these 3 eras.