If you have a route that makes use of portals, what are some nice ways of 'masking' the fact it is a portal - such that it would blend seamlessly into the rest of the scenery and not be a magical train-spawning trackend, other than the 'tunnel in the side of a mountain' approach, especially in cases where a mountain would be laughably inconceivable, such as a port or wide, flat tracts of land?