As we are modeling the real world and do not have space to model everything, we need to develop “scenes” that we decide to use. Two adjacent scenes may not have much in common so to reduce the jarring effect of this change a “scene divider” is used. A road bridge pictured below can be an effective divider. I especially like this road as it looks very realistic, very well worn, with missing divider paint and cracks in the surface. Clearly, it is a well-used bridge as there is a lot of traffic on it already.
Another obvious scene divider would be a mountain. As I am modeling in the Western US, this is clearly a realistic option. One other use of a mountain would be to hide a non-prototypical curve if you had a sharp corner that has no purpose other than to get a train through 90 degrees on your layout.