Basically, unless you want to spend large amounts of money (which you don't need to), your options are GMAX or Blender - both free and both have exporters for Trainz. Each has it's good and bad points.
GMAX is older and no longer supported, but some people prefer it.
Blender is current, Open Source, and being further developed. The "production" version (2.49) can be a bit of a pain to use until you master it due to the 'odd' user iinterface!
(There is a new version (2.5) in development (Beta test) with a better UI but no stable Trainz exporter and little usable documentation at the moment).
A search of the forum, or Google, for either of these will give you more details.
