You can't re-skin objects "in-game". What you can do is download a free resource called dxtbmp. Flight Sim re-skinners use this a lot, and it works in Trainz too. What you do is you find the files you wish to re-skin, open them up in dxtbmp and then send them to your paint editor (MS Paint, Adobe Photoshop etc.) You do all the necessary painting and stuff in there then when you are finished, save it and click 'Reload After Edit' in dxtbmp. You then save your work in there. Sometimes you have to experiment with different formats (eg Targa with Alpha, DXT1) if you get just a white texture showing as it is usually to do with alpha channels not being assigned properly.
If you need more help, let me know and I will try and dig out some info for you.
Best wishes,
Padster