That's PBR textures you can eliminate this by setting the Shader-quality setting to Standard. PBR textures work well with PBR textured assets and do not mix well with non-PBR textures. This causes those voids under the tracks and roads and buries these in other places.
I found that PBR textures are great if I start with a new route but upgrading a very old route is painful. By setting the shader quality to standard, I avoid the buried track and roads, and above all eliminate the jelly-effect caused by the PBR textures when they are placed on steep angles.