I think we beat this to death about 3 or 4 years ago Walt, and basically determined it was a term mainly used here in Maryland and surrounding areas.
From Baltimore to Cambridge, and all the way down here to Berlin and Ocean City, we find ourselves putting up our "train gardens" at Christmas. Been that way since I was a kid, and I am 65. It probably was that way long before I was a kid, just a local thing I guess. That is what my father and his friends called them.
I find that when I post I use layout more and more and have moved away from using "garden." And we mostly seemed to associate snow scenes with Christmas gardens over the years around here.
Here is mine from this year. Hope this helps, Greg
Link to more of layout......
Train Garden 2012