Hi Dawe10
HD Terrain is progressively reduced in detail/resolution as you move away from it, so as to avoid performance issues (this is the equivalent of LOD/'level of detail' systems, to reduce detail, on other object types in Trainz). To do this, Trainz has to 'average' the height of each vertex as it reduces the detail; if you have a very narrow, tall, 'bump' in the terrain (ie a tall, narrow, embankment) then as it reduces the detail this may become narrower or lower. Generally this won't be too bad on most routes (since embankments will tend to be wider at the base, and generally on less even terrain), however in this case as you have a fairly extreme example it may become lower/smaller in the distance. If you do believe this is an issue in a completed scene, then please submit feedback via the bug report form above so that we can look into it.
Regards