I think every modeller at some stage would like a tunnel on the layout. The can make very good scenic breaks on end to end layouts, and add interest for the viewer. The technique is very similar to that of  landscaping, but there are a few points to note.

Make sure that you can reach all parts of the track for cleaning and maintenance. Make sure there is sufficient clearance between the walls of the tunnel and your rolling stock. Paint everything inside the tunnel black. There is nothing worse than looking into a tunnel and seeing bare wood.