Create a timber tunnel portal over a single N scale track. Color with Earth Colors(tm) Liquid Pigment. Outside: 2 5/8" w x 2 11/16" h (6.66 cm x 6.82 cm) Inside: 1 5/16" w x 2" h (3.33 cm x 5.08 cm) About Terrain Structures Tunnel Portals, Retaining Walls, Culverts, and Roads reinforce natural terrain contours and add functionality, realism and interest to your layout. WARNING: CHOKING HAZARD - SMALL PARTS! NOT INTENDED FOR CHILDREN UNDER 3.