Terrain Transparency

I have a composite 3D model with terrains, roads, pipe networks, and etc.  The terrains have materials applied (draped rasters for the existing terrain and the "Grass" material for the proposed terrain). I would like to be able to use the "Transparent" display style so that I can see the pipe networks through the terrain.  When I choose the transparent display style, however, all of the elements except the terrains become transparent.

I did some tests in a separate file with a simple solid and simple terrain model and I got the same result - the solid becomes transparent but the terrain does not.  Am I doing something wrong or is this a known issue?  Is there a way to make terrains with materials applied transparent?  Any feedback is greatly appreciated!