Microsoft Paint? No. At least not without jumping through more hoops than needed.
Download and learn how to use CS2 (there are hundreds of tutorials for this on the web). Then when you have a hang of the tools have a go at repainting a wagon. The Golden Age of Rails has some great repainting templates for their rolling stock.
http://www.adobe.com/downloads/cs2_downloads/index.htmlYou will also need the DDS plugin from Nvidia:
https://developer.nvidia.com/nvidia-tex ... -photoshopRSBinTool translates RW texture files into DDS files, and then allows you to import DDS files back to RW texture files.
Remember to back up your work often (not the same drive if you can help it) and use layers when adding new details to your repaint in CS2.