If you're hand spiking on wood ties....homasote
If you're laying flex track.....homasote or cork
If you have no problem glueing track down (no spikes/nails)...foam
I still prefer plywood subroadbed(although I do see a lot positive about spline roadbed), on risers, on open grid benchwork/ homasote/ flex track, balasted, and weathered..................but then, I remember sawdust, coffee grounds, and lichen, as primary scenic materials.
Pete