For train terrain, I’ve often seen them put a thin layer of Sculpt-A-Mold over the surface of the hard board, let it dry (gives you a rough surface to grip to), then after your plaster mold has set up enough to “gel” but isn’t totally hard yet, you can flip it over onto the board, and let it finish drying (leaving the mold in place until it finishes setting up). It should follow any contours you have on the board – which should work well with your tunnel. The tricky part is the timing, I think. If you let it set up too much, before hand it may crack. Too early, and it’ll still be too sloppy.

One of my model train sets has a hardboard background that is in place to support the tunnel side wall and I need to stick the rock face molds onto the hardboard so the side of the tunnel (mountain) starts to look like rocks.. and idea what kind of glue I can use? I tried wood glue but the Plaster of Paris just pulls off… Thank you…

Advice? Well, I’ve not stepped into a new gaming group that I wasn’t just creating myself often. So I can only speak of the one time I did that. My advice is to watch and listen more than you speak and ignore. Gaming groups sometimes have some unspoken rules and ways they do things. But by all means jump in and get involved. Gaming is a grup activity for a reason, players (including the GM) need to bounce thoughts and ideas and actions off one another. That’s the beauty of this game.