I used the following method to add a simulated rock texture to the interior of my tunnels, and on the concrete stairs at each end of my layout:
First, crumple a piece of heavy-duty foil and then carefully spread it out. Spray it with Pam. I prefer to use the non-stick foil + Pam.
Next, mix up some mortar and add a small amount of concrete colorants. You can get these at Home Depot, Lowe's, etc. Some types are powder, some are liquid. Use the colors sparingly - it takes very little to get a nice rocky color. The choice of color will depend on the rocks you are trying to match, and whether it's supposed to look like natural rock or broken rock (such as a tunnel interior).
Spread the mortar onto the foil about a quarter to a half inch thick. Wet the surface to which it will be applied. Then carefully lift the foil by one end, and slap it onto the surface. Press it in place, making sure the mortar makes good contact without any air bubbles.
Let it set. The time required will depend on the type of mortar, how thin you mixed it, and most of all, ambient temperature. In hot weather it will set a lot quicker. On a cold day, it may take hours.
Then peel off the foil. It's possible to let it set completely hard before removing the foil, but I find it's a bit easier to remove when it's at the "green" stage - solid, but not rock-hard.
Bob Treat used a slightly different method to create rock mountains for his Snow Creek Railroad. In a nutshell, he used two layers of foil, laminated together with spray adhesive. Then he crumpled it into rocky shapes, laid it into a hole in the ground for support, and added the concrete/mortar. He let it harden, removed the foil, then cemented these artificial "rocks" together to build up a mountain.
