Create a cut stone look for your double track, HO scale tunnel. Color with Earth Colors™ Liquid Pigment.
 
Outside: 8 1/8 in W x 5 1/8 in H (20.6 cm x 13 cm) / Inside: 4 23/32 in W x 3 27/32 in H (11.9 cm x 9.76 cm)