The shortest connection between A and B is still the straight route. It's clear that sometimes this can only be realized with the help of a tunnel. This applies in equal measure for the example such as for the model. It's the best to think about setting model railway tunnels already by planning. In addition, you have to decide, whether the tunnel route will be single or double track, the tunnel portals will look modern or classic, whether a catenary will be plugged and so on. No matter how your personal tunnel will look - it is always the same procedure.