Make a fancy static shape tree. Create an animation that moves the tree at constant speed in a specific direction for a long distance (from out of the view, past the train, to out of the view again).
Spawn loads of these trees, and play the animation in a loop.
Make the ground texture move with the same speed for even better effect.
This would likely be the best way to do it.
Perhaps different animations to go at different speeds, such as 40bps, 50, 60, etc.
I wouldn't mind seeing this done, along with maybe some other models such as mountains that can be placed further back and slower to give a better illusion.
