Most tunnels, but not all, are installed the same as rail. Chose the tunnel you want to use. Click on the terrain where you want the tunnel to start move cursor to where you want it to end and click again, Then attach your track to the tunnel ends. If you have a mountain, place the tunnel ends on opposite sides of the mountain and the tunnel will go thru it. Or you can lay out the tunnel on level ground then use the raise terrain tool to create the terrain over the tunnel.