Scratch Building Tunnel Portals for a Curved Tunnel

Model Railway Train
B&M Noskowski/Getty Images
  • 01 of 07

    Scratchbuilding Tunnel Portals

    testing the portal
    Ryan C Kunkle

    Lots of model railroads have tunnels. Aside from representing prototypical scenes, tunnels are visually interesting and can make a layout appear larger, or divide neighboring vistas. There are lots of tunnels and tunnel portals available on nearly every scale, making it easy to add one just about wherever you choose.

    For this installation on the Rio Grande project layout I wanted a simple pair of concrete portals for each end of a short tunnel on one end of the railroad. This tunnel provides an important visual break between the two sides of the layout. Despite being a less-common scale/gauge, there are commercial HOn3 portals available. But this is more unique. These same techniques will work for other scales as well.


    Something as simple as a concrete tunnel portal is an easy place to save a few dollars on pre-cast parts. These fixtures are easy to make using scrap materials from other projects. Scratchbuilding also affords the opportunity to make something a little different from those that come out of the store.

    Coping with Curves

    Scratchbuilding will also help cope with one more issue facing this seemingly simple tunnel—a curve. Most commercial portals are designed to provide adequate clearances for equipment on the tangent (straight) track. On a curve, width requirements change. On tight curves like those associated with Colorado narrow gauge tracks, the dimensions change even more.

    Steam locomotives, with their long rigid wheelbase, will experience greater overhang on the outside of the curve. Longer rolling stock, like passenger cars, will have a greater overhang on the inside of the curve. Troubleshoot by testing your clearances using the largest cars and locomotives you have on your layout. Articulated steam locomotives, passenger cars, intermodal flatcars, and auto carriers are common choices. For the Rio Grande, it was K-27 No. 464 with her wide snow plow and a passenger coach. Don't forget to check the height too!

    It is a good idea to build a test portal first. Follow these steps to build a rough portal first. Build your skills, determine all of the critical dimensions and then duplicate the results in finished form.

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  • 02 of 07

    Cutting the Arch

    portal template
    Ryan C Kunkle

    The tunnel portals for this project are made from scraps of 3/4" plywood. The size of the wood is not critical. Make the overall piece slightly wider than necessary so you can bury the ends of the portal in the scenery when finished. Like the prototype, you are building the tunnel to match your particular scene. Each installation can be a little different—that's one of the advantages of building your own.

    Most tunnels feature an arched top. You can cut this rounded edge carefully with a jigsaw, bandsaw, or scroll saw. You can get faster and smoother results easily with a drill, however. Using a key-hole, spade, or Forstner bit will produce a smooth circle of almost any diameter. Smaller scales have an advantage here as bits above 1 1/2" diameter cost considerably more.

    A 1 1/2 inch hole would have worked perfectly for HOn3 if the track was tangent, providing just enough clearance. With the curve, however, the bore was increased to 1 5/8". A drill press is a very handy tool here to keep the cut perfectly vertical and straight. You can use hand tools as well, but be careful to keep everything aligned.

    Locating the Hole

    Mark the centerline of the tunnel on the board. Next, make a mark on the centerline at the apex (top) of the tunnel. For your first test piece, make it a little higher than you think you will need—it will be easier to make it shorter later if necessary.

    Remember to consider roadbed and benchwork when calculating height. The portals made here were made 1/2" taller so that they could be fastened to the sides of the ​​subroadbed.

    Once you've located the apex, measure down one half the diameter of the bit to mark the center. For the 1 5/8" bit, this is 13/16". Line the drill bit up to this point and drill your hole. Drill from the front of the portal to the back. If the back of the wood is torn up by the bit (it probably will be) it won't be seen. Drill completely through the wood.

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  • 03 of 07

    Portal Sides

    cutting the portal
    Ryan C Kunkle

    With the arched roof of the portal complete, all that remains is to cut the side walls. Mark the sides with a square, connecting the base of the portal to the circumference of the hole.

    If you are making a portal for a tangent track, a straight cut is all that is required. For curved tunnels, you'll need to add one more step. The wall on the inside of the curve may be cut perpendicular to the facade. On the outside of the curve, cut the wall at an angle away from the facade, making the portal wider at the back than the front. The exact angle will depend on the radius of the curve. A 30-degree angle will work for most installations. Blend the beveled wall into the arc, sanding if necessary.​

    After making the cuts, test fit the tunnel. Trim any additional materials as necessary until you have all the room you need.

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  • 04 of 07

    Adding Texture and Detail

    adding texture
    Ryan C Kunkle

    After all of the cutting and filing and testing, the rest is just a matter of modeling. These portals are designed to look like poured concrete. Concrete that has had a few years of age.

    First, a cap was made from 1/8" x 1/16" basswood along the top of the portal, about 1/2" above the top of the bore itself.

    To simulate the texture of concrete, a thin layer of drywall plaster was applied over the face. You can also use the plaster to help hide any scars from cuts on the inside of the portal. Just remember, you're also making the portal a little bit more narrow, so make sure you are meeting the minimum clearance. A little goes a long way. Mixing the plaster from the dry mix will allow better control over the consistency. Allow the plaster to dry completely before painting.​

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  • 05 of 07


    Painting the tunnels
    Ryan C Kunkle

    Paint the face and inside walls of the portal with thin washes of acrylic paint. After applying two or three coats to build up the color, allow the paint to dry completely.

    For some more variety and tone, a few drops of raw umber can be added to tan paint and another wash applied. This creates some nice streaks and subtle color differences which add greatly to the overall look.

    Paint the back (inside) of the tunnel flat black using inexpensive spray paint. (Always use spray paint in a well-ventilated area.) Spray a little along the inside edges as well, focusing on the top of the arc. This will provide a nice weathered look on the walls of the portal. Additional soot stains will be added to the face after installation.

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  • 06 of 07


    Installing model train portals
    Ryan C Kunkle

    The tunnel portal can either be installed on top of or around the subroadbed of the benchwork. If you have a solid tabletop, glue and nail or screw into the bottom of the portal. If you have an open-grid or L-girder style benchwork, where there is a base under only the track or other select areas, you can use the sides of that subroadbed to both support and align the portals.

    If your subroadbed is wider than the width of the portal, cut 3/4" wide notches in either side to that width. Here, that worked out to the width of the cork roadbed. A jigsaw works well to make short work of this step. Keep the cuts perpendicular to the right of way to help maintain the alignment. A jigsaw works well for this step.

    Lastly, before you install the portals, apply a quick coat of black paint to the track and subroadbed inside the tunnel. Clean the tops of the rails and you're ready to install the portals.

    Do a dry fit first to make sure no last-minute changes are necessary.

    Fix the portals in place with wood glue and a small brad or two from a finish nailer. Throughout the process, check to make sure that the portal remains perpendicular to the centerline of the track and plumb to the ground.

    Before the glue has time to set, check the clearances one more time!

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  • 07 of 07

    Finishing Touch

    The finished portal
    Ryan C Kunkle

    Now you can finish the portals with a little weathering on the face. Most tunnels and underpasses have a tell-tale black streak from soot and exhaust above the centerline of the track. This is easy to add via a little black weathering chalk. You can add as much or as little as you'd like. Make sure the paint has completely dried first for best results. The drywall mud is porous enough that sealing the powders should not be necessary, and the portals will not be handled much from here on either.

    With the portals in place, they will serve as an anchor for a tunnel liner inside the tunnel and scenery on the outside as well. Even without the surrounding scenery, just these pieces make your final layout much easier to visualize and should help motivate you to keep going!