Christopher Brimley updated September 21, 2011

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  • No-Sweat Spline Roadbed

    by William Mitchell

    Photos by the author

    A pair of Stewart CP Rail C630s head up a string of hoppers testing a newly laid section of track on the authors Sicamous Junction layout. The spline sub-roadbed the author is using was made from fiberboard.
    Model Railroading - June 2001 - Page 32

    I had decided on fiberboard (soundboard, untreated TenTest, ceiling tile) as the flat roadbed for the yards and industrial areas on my new layout. It is cheap, light, holds rail spikes well and cuts easily with a utility knife. As one of its names implies, it is a very quiet material to run trains on. I had to construct a 102' long climbing, twisting grade to provide the connection between the two levels of the layout. One night while cutting fiberboard for the lower level, I wound up with a narrow 8' length that I was flexing in my hands while thinking of how I was eventually going to construct the ascending grade. In one of those Eureka! moments, I flexed the fiberboard into a circle and knew I had my spline material. It had been sitting in front of me all along. A 4x8 sheet of fiberboard will give 24 eight-foot lengths 2" wide. With a thickness of 7/16" , this means that six pieces would give me the 2" total thickness I wanted. That worked out to 3 sheets for the spline structure at $6.00 a sheet, which is mighty cheap construction.

    A metal straight edge, utility knife, rasp, drywall screws, latex glue, a drill and sander of some sort are about all that is needed to create the fiberboard splines. A thick glue such as No More Nails® or Liquid Nails® must be used to keep the vertical joint surfaces in place while positioning the spline lengths. I found traditional white and yellow glues too runny. Drywall screws have coarse, deep threads that hold well in the fiberboard and can be bought in bulk to lower costs. Keep the screws pointing into the middle splines so you dont have the needle sharp points exposed on the outer surfaces.

    A few words of caution regarding this material. There is a minimum radius to which it can conform without snapping. I had no trouble on 30" radius curves, but I dont think you are going to get 18" curves with it. Dont wet it or it will turn to mush. Getting energetic with the drywall screws will drive them right through the material. Put the drill on slow speed and just snug them tight they will create their own countersinks. The screws eliminate clamping and allow the work to progress very quickly.

    Start the roadbed on a straightaway, which is hopefully also the beginning of the spline structure. On my layout, this was at the 8" high end of a 32' long Styrofoam 2% grade. A wooden riser supports the underside of the splines so that the top is level with the top of the Styrofoam grade. A second riser positions the outermost spline even with the outside edge of the Styrofoam. To get started, No More Nails® was smeared on the face of the Styrofoam, on the end of the first spline, on top of the supporting riser and on the inside face of the outside riser. The spline, which was then fixed in place with a drywall screw against the outside riser after making sure all glued surfaces were in full contact with each other. The other five splines were cut at their far ends so that a stagger results with the next course of splines. Their near ends were smeared with glue, wiggled into position to spread the glue and clamped to the previous spline with a drywall screw. I only glued the joint areas of the splines on straightaways, binding unglued sections together with the screws. This allows for the easy separation of splines if you build turnouts into the structure. Splines on curves were smeared with glue for their entire length to provide support for the filing of super-elevation into the curves.

    Splines must be staggered at joints for maximum strength - the farther apart the better. Glue was not applied in this photo for clarity. Note the screw tacked into the top of the wood riser to hold splines in proper alignment.
    Model Railroading - June 2001 - Page 33

    As with any splining material, stagger the individual interior joints for maximum strength and avoid joints in a semi-circle. It is some weird law of physics that a material can be very flexible in the middle of a length but almost rigid at its ends. This is useful in that it will automatically build easements into curves without you having to think about it. Plan ahead so that both ends of a spline are in the straightaways on either side of a semi-circle. A joint in the middle of a semicircle will result in a wedge rather than an arc. Clamping to force an arc will fracture the fiberboard. The inner joints are under compression so start on the inside of a curve and work out. In a sixspline sub-roadbed, the three inner layers under compression in a curve are built up first to support the three outer joints under tension. This material will fail under tension if not supported.

    All exposed mainline curves on the new layout are super-elevated. This is easy to build into the curves by fixing each spline slightly higher than the preceding one as you work outward. When the glue is set, the stepped material is smoothed with a coarse rasp. Low spots are patched with PolyFill and then all is blended with a palm sander using the coarsest grit disc you can find. Wear eye and respiratory protection if you use a power sander because the air will be filled with fine dust.

    A palm sander is used to blend fills and transitions. The dust collection bag on the sander really helps to reduce airborne particles. Note the end of the spline structure in the background in a butt joint with the upper level.
    Model Railroading - June 2001 - Page 34

    Construction is fairly straightforward using splines. Again, plan ahead so that the two outermost splines have the turnout roughly in the middle of their 8' lengths. These outer splines will be under compression once you force the divergence and will support the inner splines under tension as you build them up. Using a turnout as a template, the three inner and three outer splines are forced apart with needle-nose pliers to the proper divergence and secured in this position with a 3" wood screw. The beauty of this method is that if you are handlaying track, you can set the divergence to any frog number you want as well as create curved turnouts. The splines are added working inward with the joints staggered so that eventually six splines are built up in both the through and divergent routes. Fill in gaps with PolyFill; sand when set and voila - custom turnout sub-roadbed.

    Cork roadbed is glued to the splines with latex contact cement. Solvent-based contact cement is faster and stronger, but I dont want to be breathing in solvents while gluing. Also with Styrofoam here and there on the layout, I dont want to watch Styrofoam disappear in front of my eyes if I accidentally dab some solvent-based cement on the Styrofoam. The cork roadbed is used only on what I classify as mainline track. Yard and siding track was fixed directly to the flat sheets of fiberboard with standard rail spikes. These spikes are too short to go through the cork and into the splines to secure track on the mainline so I tried double-sided tape for the first time and am quite satisfied with the results.

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