1. Obtain Blender or Sketchup both free, or 3DS max 3200 USD - These are apps that have export to Trainz capability - Obtain the exporters - install them.
2. Learn how to use the software for editing the models - making the models as low Poly as possible
3. Learn how to texture the models - UV mapping phototextures made in Photoshop or GIMP, or applying pre-set materials in the modeler apps, or phototexturing in sketchup > in sketchup not too hard once you've practised a bit.
4. Learn how to setup the models for export > export the model
5. Import them into trainz with the content manager
All of the above takes alot of time to learn. Unless you're using Sketchup, which will take some learning but alot less time and or money.
OR, ask someone here who's into vehicle modeling to make them for you
Rico