There have been several topics about this over the years. I know there are some creators who have done it, but as I understand it, unless you have the source files (the mesh), it's not really possible. From what I've heard, even if you have the mesh, it's still not worth the time it takes to convert them.
Well, the only legitimate way to convert Msts stock over to Trainz format, is to have the original mesh and also the permission of the original creator to do it.
All the below were conversions I did of my Msts models over to Trainz format :-
In re-reading my post, I realize that I should have worded that differently. Your models look great and I wasn't suggesting it was a waste to convert them. I'd probably do the same if I was involved with MSTS as well as Trainz.
I was sort of assuming that the original poster did not have source files or content creation experience. As such, conversion would certainly not be a simple process and in my estimation, it would probably be faster to learn the art of content creation from scratch.