Tunnel portals are present on almost every model railroad, so we need to know how to make them. MRA Contributing Editor Martin Tärnrot has previously covered how to scratch build stone tunnel portals, with bricks engraved into styrofoam. Now Martin shows how to make a concrete tunnel portal, which is very easy to cut, unlike stone portals. The trick with concrete portals is in the paint.
MARIAS PASS TUNNEL 4.0
Martin has selected Marias Pass Tunnel 4 for this project and printed out a photo he found online. Before printing, he scaled the photo to size considering the standard clearances needed for HO scale. Fold the printed image in the middle to make sure the portal opening will be symmetrical. Now he has an outline template for the tunnel portal, which he puts on the material he’s selected, typically styrofoam, in this case 3/4-inch (16mm) thick styrofoam.
Cut out the outline of the tunnel portal using a hobby knife. Draw the portal opening and use a jigsaw to cut it out. The text “TUNNEL 4.0” is molded into the actual tunnel. This can be made in a number of different ways. Martin 3D printed it and shows how he assembles it.
After TUNNEL 4.0 is in place, it’s time to put putty, which gives a stone-like texture, on the portal’s surface. After the putty is dry, sand all surfaces flat. Next, engrave the pattern of the molded concrete. Press the engraved sheet into the styrofoam, then give the tunnel portal a spray of water-based acrylic gray paint.
CREATING STREAKS IN CONCRETE
Now it’s time to begin creating the vertical streaks that are typical of concrete construction. Martin shows in detail how he creates first white streaks, then reddish-brown, then black, each based on where these color streaks usually appear in concrete structures.
The tunnel portal is ready for assembly in the landscape.
For another take on model train tunnels and portals, check out this video with modeler Tony Koester.