So, in brief, it really is a dynamic environmental shadow. That implies there is some kind of rectangular mesh object somewhere up in the sky casting that shadow.
If it was my problem, I would set the World Origin to 0 degrees latitude and the time of day to 12 noon. The sun should then be directly above the shadow. Using the free range camera (or whatever it’s called in TRS22) I would go to the shadow at ground level, look up towards the sun to check if I can see the object. Of course, if it is just a plane with a 1-sided texture on top, it would not be visible from below, but it’s logical to check this first.
Next I would point the camera directly down in the centre of the shadow and gradually zoom up, hoping to see the object when the camera gets above it, but before the view transforms to the mini-map view (does TRS22 still have that?).
If that fails to find anything, my last resort would be to delete all mesh-based objects (scenery, splines, locos etc) from the area in and around the shadow to see if that removes the shadow. Since the origin point of the mystery object isn’t known, you don’t know where exactly to point the deletion cursor. So, I would use the copy/paste tool to copy a large area of the map that is known to be empty, and paste it in and around the shadow to delete any objects directly above or below the pasted area. You might need to read up on how the copy/paste tool works if you have not used it before (it’s not exactly obvious).