Homasote

[TrainNut]

From their site...

Homasote is flexible, versatile, tackable and easy to cut with a mat knife or saw. That makes it ideal for numerous hobby uses. For example, Homasote makes an ideal base for model trains;

See, there you go... it's official! According to them, I can get it at Home Depot. I thought I tried them and they told me to go jump in a lake... well, not in those exact words.