This tutorial is a long one but goes into all aspects of creating Speedtrees for Trainz. From setting up your tree lighting, materials, color sets, LOD and wind to creating single trees or trees from a Speedtree library where each new tree references the same textures saving on performance and memory. All config.txt setup is explained and also full seasonal support.
Speedtree Tutorial Files are HERE. Please not they are the tree assets referencing a tree library as that is the best way to create your Speedtree Assets for Trainz.