Willi has already made a very detailed explanation of modeling that has proven to be very informative/instructive. I’ve suggested ways of texturing rolling stock in other areas of this forum.
I can’t think of anything that I can say here that would add in any substantual way to either of those threads.
@ Peter: On my forum you have written that you work with Gmax. As I have answered there, my Gmax has only plasma(.P3D) export format and I have no program that can read this format. So how do you convert your Gmax files to X format?