Modified the add_flag section - should work ..
If add_flag TFormPoint -1,.01,0,bike, 0 AddVertex trail_surf,TFormedX(),TFormedY(),TFormedZ() TFormPoint 1,.01,0,bike,0 AddVertex trail_surf,TFormedX(),TFormedY(),TFormedZ() AddTriangle trail_surf,trail_vert,trail_vert+2,trail_vert+3 AddTriangle trail_surf,trail_vert,trail_vert+3,trail_vert+1 AddTriangle trail_surf,trail_vert,trail_vert+3,trail_vert+2 AddTriangle trail_surf,trail_vert,trail_vert+1,trail_vert+3 trail_vert=trail_vert+2 Else TFormPoint 1,.01,0,bike,0 VertexCoords trail_surf,trail_vert,TFormedX(),TFormedY(),TFormedZ() TFormPoint -1,.01,0,bike, 0 VertexCoords trail_surf,trail_vert+1,TFormedX(),TFormedY(),TFormedZ() EndIf