Some people use indoor-outdoor carpeting (very tight weave) and some others use 2" or thicker foam board. Search this forum (or the web) for tips on which product to use and why the user chose that product. If you still decide to special order Homasote (and many here would go that route), order just enough to go under your track. Homasote can be cut fairly easily (although quite messy), and using it for scenery base is really just wasting money.
Chuck