Multiple Alternate Surfaces

You are probably aware of this, but you don't need to specify the complete design surface in the template. Depending on the design stage settings, the top-surface is created for every corridor even when no alternate surfaces are specified. To my knowledge alternate surfaces are only needed when you need an alternative to this default top-surface.

EDIT: As Shawn pointed out below, the top-surface (exact name: Top mesh) that is created is not a true terrain model. Some tools recognise it as a surface that can be used in the same way as a terrain model, e.g. Sight Visibility or the profile creation tools. Other tools such as Aquaplaning apparently require a true terrain model.

Correct, requires one addition step to create the terrain from Design Mesh... The Terrains from Meshes tool can set specific Feature Definitions for the proposed terrain (e.g. boundary, void and surface) and  "side of closed mesh".   Top and Bottom can be done, however the Terrain Filter and alternate surfaces methods provide more control for pav only and subgrade terrain development.  Another possible benefit to the Terrain Filter method is how it remains dynamic to the 3D linear feature changes produced by corridor updates. 

Terrains from Meshes produced using this tool do not seem to remain dynamic to the corridor mesh changes, despite the mention of "ruled" options in the tool settings dialog.  This suggests you'll need to use the tool again to reproduce the terrain each time the corridor design changes.