Create Terrain From Graphical Filter
Graphic Filter Manager
Filter Feature Type = Break lines
Civil Features selected include mesh that is to define terrain. The mesh is created from a template for a road that has points that run along the top of the surface and points that are directly below those points.
Sometimes the top of the mesh is used and sometimes the bottom is used.
In the attachment, the green shapes represent the pavement and the red lines represent triangles of the terrain that gets created.
A terrain model is a surface where you can't have same XY with different elevation. It means that the triangulation includes one or the other. There is nothing that tells the software to pick the highest. So you should remove meshes to only keep what must be included. Sometime you can also set alternate surface to points of the template to get required features in a terrain model.
Use of Alternate Surfaces crashes when I Create Terrain From Alternate Surfaces.