first, you need a paint program, like photoshop or something, and then you need to download (well you don't NEED to but its much easier to) a template for the track style that you want
here. Open the template in your paint program, and make the cartexture that you want. Save the file as a .tga and then open up the track that you want the cartextures to be in, go to - coaster --> settings --> trains --> texture - and then choose the cartexture that you saved. If you want to apply it to more than one train, then select the train above the cartexture option, and apply it to that, too.