It has the basics. Your desired NYCTA subway train will use a different texture file but the process is the same. Do the tutorial first then attempt the subway car. Be aware not all assets are suitable for simple reskinning, sometimes the content creators use more complex approaches that may save on texture size but are much more complex to reskin.
The Trainz Resources Directory lists a few skinning tutorials. JoshEH has done one for Photoshop, but it can be applied to GIMP. All Ricks Trainz has done one for MSpaint and it can also be used. There are also others.
