Ok, what is a spline roadbed?
This is the first I have heard of it.
How is it made / used? What are is pro's and con's?
Spline roadbed is made with thin strips of wood glued together in layers. The thin strips can be easily bent to follow the shape of the track. So one strip is curved into place where the track is supposed to go, then the next one is applied. Since the inside of a curve is less distance, once the strips are glued together they hold that curve. Most often strips aren't glued one to another but to a spacer block between each strip. This lowers the cost and weight.
Spline was poplular back when plywood was expensive and one could get oak pallets for free. People would take the 1x4's off and rip them into spline strips.
Advantages:1. Anytime something is made in layers the strength is increased, so spline is very strong.2. It is easy to make spline follow the exact lay of the track and make junctions.3. Because of #2, and the offset of the strips of wood there is no loss of strength when tracks diverge. Spline is very strong.4. Since a single 1x4, 1x6, or 1x8 can be sliced into many pieces of spline it can be very economical.5. Because the spline bends side to side but not up and down it is hard to make a transition to a grade that is too steep.
Disadvantages:1. Making spline is a lot of work.2. Making spline is a very slow process (the more "C" clamps one has the faster the work can go).3. Spline is very unforgiving when a change in the trackwork is desired. More spline can't just be added, it has to be spliced into (assuming one wants to retain the strength).4. Making the transition from flat track to grades is difficult.
Other:Unless one has lots of time, lots of saw blades, and very little money spline is the way to go. I've found it much quicker and easier just to use 3/4" plywood cookie cutter. The complexity of making a few joints is much less trouble than spline.
Gandy Dancer, your "disadvantage" about the transition from level to grade is incorrect. With respect, that is precisely one of the chief advantages to spline roadbed in the first place, the fact that easements up or sideways are so easily and smoothly attained. What you do is start on the level, anchoring several level inches of splines, and then force them all to begin a slow curve in the direction desired by laying them atop risers screwed to the cross-members/joists, on the benchwork. Each individual slat is wobbly and willing to bend to an extent, sort of like spaghetti pieces dipped a few seconds into boiling water. So, you lay two or three of them side by side, clamp them to the risers and to each other, and you keep pressing them into a flat roadbed at the top until the glue begins to set. With enough clamps, you only have to clamp them into position and walk away to let them dry. When you come back 60-90 minutes later, they are like redried spaghetti...essentially rigid. You finish widening this roadbed by adding another series of spline sections beside the first two or three, glue them and clamp them together. When you finish, you have your six or seven ply, really tough and rigid curves, and smooth running after you plane the surface to get rid of minor variations in the heights of the spline sections.
-Crandell
Joe Fugate has an excellent article on his web site:
http://siskiyou-railfan.net/e107_plugins/content/content.php?content.15
I have personally been debating between spline and foam construction. I chose to proceed with foam (using a plywood base for sound deadening) because of the quickness of construction.