RE: Drape a smartline to a terrain model

If you wish to use Frank's suggestion, you will need first export your terrain model back to native InRoads as a DTM surface.  If you wish to drape the feature to on the terrain model and then add it to the terrain model, you would use the method I described.  My method uses OpenRoads commands.  Frank's method uses native tools.  So, you have a couple of options.