I didn't use any sound deadening or roadbed material in areas that wouldn't be seen, like inside both helixes and in both hidden yards. The visible parts of the layout have a variety of different treatments for roadbed. No cork, too expensive.
I always start with 3/4" plywood as a base. In one small section, I carved roadbed out of pink foam. Another section I use Homasote cut on the table saw, but most of the layout is done on inexpensive fiberboard, including the entire upper deck. The roadbed profile was later carved into that material with carpet knives.
Rewatch the video more carefully, now that you know what to look for. See if you can spot all the different types. A lot of it has been ballasted so it's hard to see, but it shows through in places. BTW, the pink foam is covered with a layer of brown paper, and there is only about 40' of it.
I chose GarGraves for a couple reasons. I had a layout 25 years ago, built with 10 cases of it, which I recycled into this layout. So I just added another 15 cases to that. Oh yeah, the cost. If I'm going to be cheap on roadbed, I'm going to be cheap on track too. Especially when dealing with the volume I have.