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Written by Paul D. Race for Family Garden Trains(tm)

HDPE Flexible Roadbed

By Paul Race (with major input by Bill Logan)

Part 1

Paul speaking: About 2002, a Columbus-area architect named Bill Logan began developing a construction method for raised roadbed that:

  • Can be used by people with common tools and relatively little carpentry experience.
  • Makes curves and easements almost as easy to install as straight paths.
  • Can be installed over existing landscaping and between established plants without difficulty.
  • Creates flexible but sturdy support for right-of-way.
  • Uses pre-site fabrication with little on-site cutting required.
  • Holds ballast well in in-ground installations.
  • Gives above-ground installations an attractive, "semi-finished" appearance from day one.
  • Eliminates waste besides sawdust.
  • Makes changes and additions easy and waste-free, so mistakes don't cost you money.
  • Elimates risk of damage to the structure from warping, rot, or insects.

In 2002, Bill was on the committee to design a public display layout for the Franklin Conservatory in Columbus. With his architectural background and love for engineering challenges, he wanted to find a better way to install raised roadbed than the "tried and true" post-and-stringer method most garden railroaders - including professional installers - had been using the 1980s. After some engineering calculations and some trial and error, Bill came up with a simple process almost anyone should be able to use to create smooth, strong roadbed that conforms to any reasonable curve (and some unreasonable ones, if you ask me).

Bill gave me constant input as I put this article together, including several passages that I will try to annotate. The construction ideas are all Bill's; my contribution was to communicate Bill's ideas in a way most people can understand.

Update for 2006

Since Bill helped me put this article together, I am aware of dozens of other railroads that have been constructed this way. No one has reported a serious problem, and many people have reported their delight with the finished product. Bill has himself has advised several homeowners and at least one shopping center on building beautiful, complex railroads that use this method. (If you ever get to the Columbus, Ohio area, check out the "Easton Express" at the Easton Town Center, not far from Columbus' airport.)

In March, 2006, I reformatted this article to make it easier to read, and I made many minor changes and corrections. I also added two photos of "test runs" on the Franklin Conservatory project. I hope soon to add example and construction photos from other projects to give readers a different perspectives on some of the tasks involved, so stay tuned. Please let me know if you need any additional information or note any additional changes that should be made. Thanks in advance - Paul

What Bill Came Up With

When Bill started his research, most raised garden railroads used post-and-stringer construction that consisted mostly of 2x6 pressure-treated lumber fastened on 4x4 posts. Although Bill was dissatisfied with the amount of waste you get from using 2x6"s, he was more "put off" by the appearance of the 2x6"s on the "finished" product. Specifically, Bill didn't like the way they jutted out beyond the tie width, or the fact that they are solid when viewed from above, which makes them look silly when you try to disguise them as bridges or trestles.

While Bill was concerned with structural integrity, he also knew that he would be working with many non-carpenters on the Franklin Park project. He wanted a method that small teams without a master carpenter could implement consistently throughout the project.

The system Bill developed could be called "open stringer;" it still uses posts and stringers, but the finished installation looks far more "open" from above. In fact, the roadbed looks something like flexible "ladders" for which the "rails" and "rungs" are prepared in the "shop" and assembled to fit onsite.

What Bill Used

Looking past traditional materials, Bill examined plastic-, and plastic-enhanced lumber-replacement products.
  • He found a company in Ohio that makes "2x4s" (actually 1 5/8 x 3 1/2", you know) from recycled HDPE (high-density polyethelyne). HDPE is what you get from recycled two-liter bottles; it is chemically similar to the sheathing they use underlandfills, so it should be pretty stable. This "plastic lumber," also sometimes called "plastic wood," is substantially more expensive than pressure-treated pine, but it is much more flexible when used properly, and has virtually no waste, compared to 2x6" construction.
  • Bill also examined plastic-and-sawdust "lumber," such as that offered under the brand-name Trex. This is less flexible, less expensive, and easier to find than HDPE components, but it is usable for Bill's "open stringer" method, if you don't want curves under say 8' diameter. (I've recently seen Trex that was installed outside for three years. It seems to lose its original paint color and fade to something like the color of concrete. And it seems to get wavy if it's not supported properly, but where it's supported properly it keeps its shape.)
Unlike "real wood, neither of these products has a "grain," which makes them unsuitable for long unsupported spans, but it also means that, unlike "real" lumber, cross-cut pieces are just as strong as pieces cut lengthwise. At the checkout counter, these will both seem to be more expensive than ordinary pressure-treated 2x6's. But, unlike building with 2x6's, there is no waste but sawdust, and you can skip buying the 4x4s or whatever you were planning to use for posts.

Bill says that per foot of installed track the "expensive" material is actually cheaper when using his system: in central Ohio, about $2.25/foot versus about $2.50/foot for roadbed made with pressure treated lumber.

How the Method Works

The following drawings show how Bill rips the "lumber" apart and reassembles it in structural members that are VERY strong, and very flexible until you decide where they should go and fasten them in place. (Note: These drawings are not remotely to scale; mostly the "long" parts have been shortened by about 60% to make the principles involved more apparent in the drawing--sort of like the Lionel "Hudsons" built on the 2-4-2 frames.)

Part three of this article has photographs of people using this construction technique outside, so if the drawings don't quite make sense the first time you look at them, you'll have a chance to see what it looks like in "real life."

Tools needed:

  • table saw or radial arm saw for cutting the lumber.
  • battery screw drill for inserting screws
  • heavy hammer or mallet for pounding the posts.
  • laser level, carpenter's level, or other level for adjusting grades
  • clamps (three minimum) for holding and adjusting roadbed.
  • hand saw or battery saws-all for cutting off posts and trimming.

Materials needed:

  • HDPE lumber - 2" by 4" HDPE or Composite lumber for stringers, spacer blocks, and posts
  • Deck screws to attach stringers to spacer blocks and stringers to posts
  • Tie screws to attach track to roadbed

One nice aspect is that all the ripping and part of the reassembly can be done in a comfortable dry workshop months ahead of the actual track-laying.

In the Shop

Rip 3/4" wide stringers off the outside edges of the 2x4 leaving a 2" wide core.(Follow the manufacturer's cutting instructions. Also, see Dave Smith's hints about cutting HDPE lumber in part 4 of this article.)
For every eight foot length of the remaining core, cross-cut fourteen 1" thick blocks; three 2" thick blocks; and leave the rest of the core for vertical posts.
Fasten 1" blocks to one of the 3/4" stringers, spaced 6" apart, using one deck screw per block. Deck screws should be galvanized exterior Phillips flat head screws 1-1/2" long specifically designed for exterior decks. (The extra density of the synthetic materials stress wood screws and drywall screws to the point where their heads break off during installation.)

A spacing and hold-down template on plywood is very helpful when producing a hundred feet or more of roadbed. This will speed the process of attaching the blocks to the strips and assure 6" accurate spacing.

A 2" block is installed in the middle and at each end (with the block overhanging the stringer by 1" for attaching to the next stringer).

In the Garden

Directly on the ground bend the roadbed where you need it to go. Use the two inch blocks to join stringers. It is important to provide for staggered joints when the opposite side stringer is attached later.

Using clamps to hold the roadbed together in place attach the opposite side stringers. Stagger the stringers. Bill recommends the use of a minimum of three "Quick-Grip - Mini Bar Clamps by Vise Grip Tools - American Tool Co. Inc. or equivalent.
Drive the posts into the ground through the roadbed far enough to prevent posts from tipping. Space 2 foot maximum. Do not sharpen the end of the post. Sharpening the post like a stake to make it easier to drive will cause the post to push out of the ground later during frost heave. A square end on the post will minimize this. You are not trying to drive the post below frost line. You are only setting the posts far enough into the ground to prevent them from tipping. Infilling with compacted landscape dirt or fill later will provide the final stabilization. The primary purpose of the posts at this stage are to adjust for grade. Driving the posts far enough into the ground to prevent frost heave will waste material and result in out-of-plumb posts and roadbed racking.
Raise the roadbed to the desired level. Clamp in place. Check grade and level. Fasten to posts with deck screws.
Once you've doublechecked the grade and the level, saw the posts even with the stringers.
Leave natural finish or paint as desired for exposed bridges or trestles. Red oxide spray primer works well. Bill recommends Krylon - Red Oxide Primer by Borden Inc. The plastic wood manufacturers do not recommend painting because the plastic does not permit paint to adhere well especially outdoors. Nevertheless, Bill finds that the Krylon primer has held up better than the rest so far.
Attach track to the Flexible Roadbed on 18" -24" centers at the spacer blocks through tie centers with galvanized #6 by 1" pan head course thread screws. (Nails are almost useless with HDPE lumber; use deck screws with coarse threads--they go faster and don't melt the lumber like fine threads will.) Always use self tapping ends.

Bill's General Comments

  • I do not recommend placing posts over two feet high unsupported between initial grade and the stringers. They are too wobbly. Provide a temporary horizontal brace from rocks, spare lumber, etc. until the landscape fill or trestle bents are installed. At Franklin Park we elevated loop #5 to 42 inches above grade on trestle bents spaced 6" on center with standard outside taper.
  • In temperature extremes, the expansion and contraction factor for the synthetic materials described above matches the polyethylene used in the prefab ties by LGB, Aristo, etc. Water does not affect the pure HDPE lumber unlike treated lumber. Repeated water exposure does cause composite lumber (such as Trex) to fuzz and pit.
  • Pre-fab track, flex track, and handmade track - all can be fastened to this "open stringer" roadbed with success. Open stringer roadbed can be completely hidden from view by rock ballast that stays in place.
  • One advantage of open stringer roadbed (similar to the 2x6 treated lumber method) is that you can construct the roadbed, lay the rails and operate the trains before the final landscape is in place or re-arranged. But, unlike using treated lumber, open stringer roadbed can be installed thru existing landscape plantings with little damage.
  • If you screw up the installation or change your mind about placement or elevation, everything can be un-done and re-done by backing out the screws to correct without waste. Try that with the 2x6 treated method.

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