a) Why not ask the original creator if he is willing to let you have a copy of the original mesh so that you can convert it to Trainz?
b) To use the program you want, needs as much ability in the use of GMax as creating a model from scratch so why not bite the bullet and learn to create your own models. Also it has limitations which would give you problems.
c) As you already know from the other thread, nobody will help you on this because of the sensitivity to its abuse for piracy (copying WITHOUT the original creator's permission).