When doing large yards, consider this: A large prototypical yard can hold thouands of cars and hundereds of locos...but in Trainz your video card will lock up and the framerates will stutter, requiring Ctrl+Alt+Del just to escape Trainz, if you clog up a large yard with lots of poly assets. Dense trees, and grass especially, adds to the poly's of a scene.
So your large yard will only be able to handle just a few trains, on it's many, many tracks. An alternative is to use "railcar splines" of boxcars and coal hoppers, which perhaps are lower poly.