What you cover the sub-baseboard with depends upon what you intend or may do with scenics. Foam board gives you the option to create holes for lakes, rivers, gullys, lower the track in an area for level rails with concrete or road, etc. You get the idea. You can hide wires in it easily for accessories. It seals easily with inexpensive latex paint. And it's light in weight and reasonably priced. You would need to install new trim boards for the sides. Check out some of Rick O.'s posts where he shows pics of his foam work. It also provides some sound deadening qualities.
If you are one of the "I have to be able to climb on it guys", then by all means, load it up with heavy stuff, Homosote or Quietbrace.
If all of the intricate contour scenicing is not in the plans, I would recommend a product like the Roberts Super Quiet floor underlayment with a layer of wood floor underlayment on top. The total thickness would be 9mm and very light with a smooth fresh surface. The sound absorption qualities of the 2mm Super Quiet exceeds Homosote or Quietbrace. A little glue and you're there. The 1/4" or 7mm wood floor underlayment would be a nice fresh, smooth surface to start anew. Not a budget breaker. Think about products used for laminate flooring sub-floors.
OK. There's my 10 cents worth. Have fun.