Woodland Scenics C1252 Tunnel Portal Concrete HO Gauge Add a concrete tunnel portal over any HO scale track. Color with Earth Colors™ Liquid Pigment. Outside: 5 3/8" w x 4 7/8" h (13.6 cm x 12.3 cm) Inside: 2 11/16" w x 3 11/16" h (6.82 cm x 9.36 cm) About Terrain Structures Tunnel Portals, Retaining Walls, Culverts, and Roads reinforce natural terrain contours and add functionality, realism and interest to your layout.