I am waiting on about 5 reskins questions from different people, that it looks if are no longer active in the game. Noething in the files say if it is okay to reskin or not, sadly. So on to next project I guess.
I've never been a reskinner although I played with PaintShed (TS4?) some years ago. Frankly, I don't know how people manage to reskin with the complex textures in use for later versions of Trainz. The TRS19 PBR materials and textures will probably kill off reskinning completely.
You might consider taking up asset creation and making your own models. It can be fun and rather addictive. In general, you don't need to ask anyone's permission to do that except for those models where copyright is likely to be an issue. Thomas the Tank Engine for example.