Steve Rosnick (MR Sept. '94) says this makes the best roadbed; avail. in building supply outlets.
It is used as acoutical panels or bulletin boards and is a byproduct of steel; namely slag.
It’s easy to cut, has negligible thermal expansion and grips spikes or track nails. Also, it is self supporting and doesn’t need subroaded.
You do need to cut or route the sides for slope and then paint the routed area with latex to seal.
I may consider this for my next indoor 3-rail layout, which I’m now pondering.