I have just used thin cardboard, old business cards to shim up the track.
I have also used a rasp to shave down some of the uneveness where the pieces join.
BOB H - Clarion, PA
I have noticed that the surface quality has gone progressively down hill for a number of years.
But that said I still like Homasote for my base!
Why using Homasote spline is better if you are using spline, its dead flat.
Dave H. Painted side goes up. My website : wnbranch.com