Since this is in Switzerland, an overland route will require a lot of ups/downs over all their mountainous terrain. A tunnel can be made to slope much more grad

Since this is in Switzerland, an overland route will require a lot of ups/downs over all their mountainous terrain. A tunnel can be made to slope much more gradually, which will increase the efficiency of the system. In addition, Switzerland has a lot of experience with building tunnels, so it's probably not quite as costly as it could be.