I"CLIP"
I cut my own roadbed, so I needed something that was easy to handle, easily sanded, and would hold the spikes well. What I have started using, after it was used on our club's original layout, is what is called insulated sheathing. Its available 2 ways. One is with a thin coat of tar applied to one side. The other option is nothing applied to any side.
"CLIP"
but the sheets are only $6.00 for a 4x8' piece. And it only takes about 2 hours of work on a table saw to turn out over 100' of roadbed. It holds the spikes very, very, well and along with the slightly thicker ties I use, holds the rails even better than when I used Homasote.
As to the tarred side, as the plain sheathing is hard to find, The tarred side is always on the bottom.