Irritating Triangular Shadows

So in TRS2019 sometimes on elevation there will be these weird shadow triangles like this,

The can also appear on the ground aswell, is there a way to fix these triangles?

In Topology Tools, use the "Plateau" (P) to smooth these out. Put the mouse cursor circle over the area of the triangle and gently move forward or sideways depending how you want the area to look, the shape will change and the triangle will disappear.

edit: Forgot to add, make sure the "Radius" dial is not too large and that the "Sensitivity" dial is not turned too far to the right or the area will move too quickly, when smoothed.

If F9 (wireframe) is used, the over distortion of the land can be seen by the sharp grid lines. When the land is smoothed the sharp lines will ease. This is just a good way to get an idea of what is making the triangle.

rs2007 - One observation I'd make about your screenshot illustrating the 'weird shadow triangles' above, is that anomalies like this are made to look much worse by the harsh environmental lighting and shadows that you have currently set for this scenario.
Suggest go to the Environment - Lighting tool in Session Edit... mode and adjust the Ambient and Direct Light RGB values to soften the shadows by increasing the ambient RGB values and reducing the Direct (sunlight) values to a uniform dark grey. (See my example below for an 8:30am timeslot on a clear, sunny Summer's day).
Shadows will become more subtle and realistic and far less harsh. You'll see details in the shadow areas underneath your locos and trees.
(The nearly black colour preview panel in the image below is my preference for setting water colours, rendering them more realistic in Ultra mode than most other settings).

I find just going into wire mesh mode and moving as in raising or lowering one of the grid points slightly usually fixes it.

Just to throw a spanner in all of the above theories and suggestions. I have noticed this frequently in my current project - always under or right next to track and always during the process of painting ground textures. Texturing is the last step in my route creation process so everything else, tracks and all scenery, is already in place.

These "artifacts", in my experience at least, only appear during the texturing process at a spot currently being textured when the texture "density" is still poor (i.e. during the first "coat" of paint). They immediately disappear when the full opacity of texture has been applied.

I have also noticed that texturing also seems to raise the ground level slightly in some areas (especially under splines) and I am not using any PBR textures, just the same ones I have used since TRS12 or even earlier. This may or may not be related.

Whether other factors are also at play here I do not know, but that is my theory and experience to throw into the mix.