The "White Stuff" is High Density Foam.
My Father "Sam" used foam underlay on his layout. It is very easy to use and not that costly, So I have done the same thing with the Main South Line layout.
The foam comes from Solartex Sales@solartex.com.au Sheets are 8' by 2' , but for transport they are cut down to 4' by 2' sheets. I cut them in to 1 & 1/2 " strips and sand the top two edges a little to remove the square corner look and make it easier to Ballast. I normal order 10 sheets at a time. Well back to work.