Boy, Andrew, patience, patience, patience....
Judging from the box on p. 48, he talks about using the "cookie cutter approach on sheets of Homasote or plywood". The pictures on p. 42 and 47 don't really look like plywood, my guess is they are homasote cut and layed on its side to make spline roadbed. Can't swear by it, but that's my best guess.
FT