I have some left over plasterboard from a bathroom refurb. Is it okay to use it on top of 1/2" ply as my roadbed for On30? It a lot cheaper than Homasote or Sundeala and I don’t just want to throw it out.
While I have never tried it, I imagine it would work, albeit quite noisier than homosote. And, you will have to seal it before laying track, as ballasting and other scenery work would likely ruin the board. (Wetness.)
I wouldn’t. Plaster board (sheet rock) doesn’t hold track nails, (or any other sort of nail). It’s brittle and easy to break while you are cutting it. The plaster core is abrasive and will dull the edge of any tool. It has no sound deadening qualities at all.
Homosote is a long time favorite. It cuts easily, takes nails well, and deadens a lot of sound. It needs to be backed up with plywood, otherwise it will sag over time.
I made my road bed from soft white pine. I used my bandsaw to resaw 3/4 inch pine boards down to 1/4 inch. Pine holds track nails well, far better than cork. I can relocate nailed down track far easier than glued down track.
I have never tried using plaster board on a model railroad, and would be reluctant to ever try it. Too messy to cut; easily cracked or broken; creates too much dust.
What are you calling “plasterboard”? Regular drywall, blueboard or waterproof/ resistant greenboard? Any of the gypsum cored products really are not a siutable replacement for Homosote. If glueing down track (no nail/ spiking), extruded foam insulation panels in the 3/8" fanfold or as much as the too thick 1/2" will work far better.
Edit: 1/2" roadbed is fine in your scale