There are a few freeware or old free versions of tree generating programs I've played with a couple, main problem is getting the polygons down to an acceptable level and find some way of importing into a program with a Trainz Exporter although I did manage to make some bushes, exported to DXF and imported into Gmax and redid all the texturing. I'd love to get my hands on Xfrog though. I've also tried the same technique a N3V used with the Treez pack again for bushes, basically a lot of flat planes arranged to give a 3d'ish effect.
Old version of Tree Generator was free or was a year ago but too many polygons, TreeD is better on polygons but pretty useless for trees unless you want them all looking pretty much the same but ok for bushes if you attack the mesh in 3ds or Gmax.