Terry
My understanding is as follows:-
TS2009 allows increased detail to be built into content and because of improvements in the code this does not have a negative effect on frame rates. In fact even with pre-TS2009 content it is reported that frame rates are improved. These new methods will not work with Gmax. Initially we were told that only content made for TS2009 using 3DSMax would be acceptable which created much unhappiness. Hence my comment.
However.
I see the current situation as follows. Content can still be made in Gmax to correct 2004/6 standards which will work quite acceptably in TS2009. The only difference is that they will not look as good when compared with content made to the new standards and they may reduce frame rates slightly. They will look just the same as they do in previous versions.
The new CM2 (a more stable, and much improved CMP) has a more rigorous checking system and will require more care when creating but it will not prevent content made in Gmax from being used.
My take on this is that Gmax will be here with us for a long time, there is no need to change to the unintuitive Blender or the very costly 3DSMax for creating content that will work in all versions. As other games also use Gmax for third-party creation I am very certain that Gmax will not disappear.
A final comment. Auran have a stock of nearly 100,000 content items on the DLS, probably most made with Gmax, they would not create a program that made all that stock obsolete. The DLS is one of the strengths of the Trainz system
You mentioned your engines. I have gathered that the treatment of steam locos has been much improved, even better than TC3 but some config and specification changes will be needed to get the best effect. Unchanged, they will still work in TS2009, mine do.
Cheers
Peter