Sorry for the belated response. :)
The textures for each car reside in your cartextures folder. Open the desired texture in whatever image editing software you have that supports the TGA file format. I recommend Photoshop but if you don't have access to it then you can download a program called Irfanview for free. Once you load the image you'll see quite readily what you need to edit. The rest is pretty obvious from there.
If you have trouble applying your texture to a train in the Editor, refer to the Technical Guide for help.
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