My tunnel portals/openings were modeled in chicken wired, then light drywall compound off my wooden "base" box for the tunnel part of my Majestic Mountain. Two Standard Gauge trains run through it; theMTH Ives Circus set, and the MTH Gold Millenium set. The straight tracks inside are close together. The box is three sided. The box dimensions are 12" deep, 12" high and 6 ft long.