A long time ago when I didn't know any better, I asked a well-known content creator if it was possible to bring in the heavyweight coaches from RW to Trainz as that's something we're sorely lacking - even today. His reply:
you cant convert one way or another. its just not that simple. each game requires different things for the models. there would be no shall we say legal way to convert those models, and it would be just like starting from scratch anyway. you have to make the geometry and re-apply all of the materials and reset them for TRS and im format.... you are just better off starting from a clean slate and making your own rather than worrying about a conversion. its just not worth the hassle.
i was able to do both TRS and RW and MSTS models at the same time because i own the models and i made them. so all i had to do was recofigure the source models for each game.