As suggested above, it's not easy.
I'll give a rundown of the basic tasks, a step by step would be pages long and take longer than my lunch break (or the rest of the day)!
Stage 1: The model
You need the original model in either Max, 3DC or Blender format.
The model needs to be updated with the mesh objects and materials fitting the recognised RW naming formats and additional settings, as do any animations.
You need to use the appropriate exporter to turn the model into an igs file and the animations into ia files.
You also need to convert all textures into .ace format, making sure they have the correct alpha settings for the shaders and the effect you want to achieve.
Note: In MSTS I believe a lot of alpha cutouts were used to keep poly counts low.. they look awful in RW unless used sparingly in small areas, also RW can handle in excess of 30k polys for a loco so these shortcuts are not needed. Bottom line here, you'll probably want to rebuild chunks of the model.
Stage 2: Blueprints
Depending on what the model is, you need to create the appropriate RW blueprints. For a simple scenery item it's probably 1 blueprint, for a loco it's more like a dozen, plus power curve files, etc.
Once all of this is together (no small task for setting up a loco, as I say scenery is pretty easy) then you can preview it in the Asset Editor, where it will most likely fail as you've missed a setting

You then chase down the errors and fix them
Stage 3: In game
You look upon what you've done and think.. why did I bother
This is a personal opinion, but I don't think I've seen anything as a direct conversion from MSTS into RW not look completely out of place. The additional graphics features of RW mean that the older models (a lot of them very simple "squared" shapes) just don't fit in with all the highly detailed, shaded models.
No offence to Mike Simpson, but compare these wagons which he converted from MSTS (in the VERY early days of RS, just so we had some stock)
http://uktrainsim.com/filelib-info.php? ... leid=18623
compared to these repaints by Brian Yeomans, which are an actual screen shot here, not a render from a modelling programme
http://uktrainsim.com/filelib-info.php? ... leid=23075
Speaks for itself really
My opinion, if you're already a modeller just start over and use the great things you can do with the RW graphics engine. If you're not, learn. I did
