Well, OK, there are so many different repaints, but most are the same to do. For example, I'll use my favorite little diesel and it's repaint by X Driver and Buzz. The GP20 by Reppo, and the PHL repaint.
In the File Library, find the repaint you want to install. Here's the PHL repaint:
http://railworksamerica.com/index.php/d ... e-repaints So, you can see it requires that you purchase the GP20 model from Reppo (an RSC partner) on the Steam games site. Download and install that. Then, download the repaint from the library here, un-zip the file (I put mine on the desktop so I can find them easy) and run the Railworks Package Manager. Package Manager is in the Railworks game folder under Utilities. (I suggest you make a shortcut to this on your Desktop so you can use it easily) Click "Install" then search for that file you un-zipped with the repaint in it. Once you have the file, click "Open".
Now you will have to find the Railworks folder on your hard drive, then find the engine. Mine is on my "C" drive, so: C> Program Files (x86)> Steam> steamapps> common> railworks> Assets> Reppo> GP20packSP> RailVehicles> Diesel. Now I see two folders. The original GP20SP, and the repaint, GP20PHL. Open the folder for the original and find the GeoPcDx file, here it's GP20_SP.GeoPcDx. copy and paste that into the repaint's folder and you're done.
Other repaints will require other engines or locomotives. Those engines or locomotives will be listed under the name of the person/ company who created it. Most but not all the "originals" are payware. So you need to own the original first, that has the GeoPcDx file. It's OK for you to use that file on your own computer, but it is illegal to give that file out to other people or to charge for it. The GeoPcDx files are the property of the person/ company that created the original.
It's not super easy, but when you do this a few times it gets easier.

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