New update: I remembered I had a disk copy of the old Rail Simulator from Kuju. I thought it had the same sd40-2 textures and models from the current train simulator USA assets and Loco Pack (available on steam), but it upon inspection, it seems my memory has lapsed.

I can now get textures, that in my option, look better than what comes with the current game aesthetically (all other locomotives and stock were more or less identical). I had to find the game in a stack of disks, download, take the sd40-2 from rail simulator, and place in the existing engine folder for the US pack, in the railworks steam folder, which then replaced the "weathered" and "unweathered" variants already in the game. I dont know how to do it without overwriting the existing sd40s because they have the same names, which make it impossible to have all 4 versions without editing the game files. The good news is that the overwrites automatically updated into all scenarios, meaning that I dont actually have to make any new scenarios to use the locomotive, given the scenarios I choose are using the "Steam USA locomotive and assets pack". Actual texture quality effected negatively a little, and some small model features are missing, like 3d radiators, and the 3d radiator fans on the top. I will assume that driving quality is similar enough that it should not adversely effect scenarios.