This innovative mold is ideal for casting realistic rock walls and ceilings for scale tunnels on your pike. It is designed to form half of either a single or double liner section with each pour of Lightweight Hydrocal. HO scale. Use a spatula to fill the Tunnel Liner Form with Lightweight HydrocalÂ®*. Join two half sections on top with a strip of Plaster Cloth which forms a hinge. Join completed units on one side for a longer tunnel. The unattached side can be raised for track maintenance.