Sorry for mis-reading this threads Question, however, it’s the very best sound board insulator, (compressed cardboard), Available. It is not perfectly level, or should I say flat. After sealing the surface with paint, when laying track, you will need to level your track to prevent dips. I used cork roadbed under Atlas O and Ross Chstom trackage, and used cardboard under the cork to level the rail. You can come to my train room, put 4 long trains, passenger and freight, and turn off all the sounds, it Quiet....Personally I do not recommend blue or pink styrofoam board, possibly homabed, but I do recommend Homasote. 
