Subroadbed Construction (Rex Beistle)

This is NOT a DCC specific topic.  However it important because it has to do with building a reliable layouts that will reliably work for a long time both electrically as well as operationally.

MYTH: The reason why our model railroad layout have track that kinks in the summer or have gaps in the winter is because the rail itself is changing length with the temperature. 

Technically it is true the rail does change size, but that is NOT THE REAL PROBLEM.  It is the wood the rail/track is laid on that is the problem.  It expands and shrinks conservatively 2x more than the rail does.   Both wood and rail are temperature sensitive.  However unlike rail, wood is also VERY humidity sensitive too.   All that is required for the wood to move is a change in humidity the allows moisture from the wood to escape into the air or enter from the air back into the wood.  While the wood is moving with humidity change, the track is standing still and forces of the wood acting on the track cause the kinks and gaps in the track.

In a low Humidity situation, the wood will lose moisture far faster at higher temperature than at lower temperatures.  So temperature is a big variable but it is a secondary factor with wood as opposed to the rail.   Once the wood moisture content has fallen to an equilibrium level with the room environment, the wood will stop shrinking.    If the average moiture level of the wood reaches a lower average level than when it was Kiln dried, there will be a "net reduction" in size of the wood.  The same is true if the average humidity is greater than the kiln dried level but instead of the wood shrinking, there will be a "net expansion".

Once the net expansion or shrinking has taken place such that moisture equilibrium has been established, then the wood will have limited expansion and shrinkage depending on the humidity changes of the year round weather which will be less than before equilibrium was established.

The following technical research was done by Rex Beistle and posted on the NCE-DCC list on 10/9/14.  It has been discussed before and posted before, but here is the most detailed version.

Rail and wood movement - temperature versus humidity.  (Rex)

The Machinery's Handbook says that the thermal coefficient of expansion of Nickel-Silver is 9 micro-inches/inch / degree F.  If I haven't made an arithmetic error, a 36 inch length of rail will expand 0.0162 inch with a temperature change from 40 to 90 degrees F (A 50F degree temp swing).

Wood moves as the relative humidity and subsequent moisture content of the wood changes.  Wood movement in the direction of the grain will move 0.1 to 0.3% as the moisture content changes.  

Taking the lower number, 0.1% along the length of a 36 inch length of timber suggests a potential dimensional change of 0.001 * 36 = 0.036 inch. 

CONCLUSION: The wood movement relative to rail movement is 0.036/0.0162 = 2.22.   Wood moves 2X more than the rail.    



1) All expansion directions considered, plywood expand the least because the construction of the layers are mechanically interlocked by glue which reduces the expansion and contraction.  (See reference 3 below)

2) Although wood is "kiln dried", the final moisture content of the wood can be far different than the humidtiy level or average moisture level of the room the wood is to be used in.  In other words for minimum wood expansion and contraction to take place, the wood and the room must reach an equilibrium in terms of moisture. 

You are better off doing one or more of the following:

1) Building your layout completely out of plywood
2) Let the given wood reach a moisture equalibrium with the room before cutting it and installing it on the layout.
3) Paint the wood exterior completely to reduce the moisture transfer between the wood and the room.

Mark's recommendation is to build the layout out of plywood and paint it white on the bottom and earth tone on the top.  White helps the light situation while working under the layout while earth tones on top helps with the scenery.

The use of AC removes humidity from the air and hence creates a very low humidity environment.  To achieve equilibrium, you need to let the wood rest for some long period of time like a month or so before you use it.  You will need to let the wood breath on all side.  

If the wood is exposed to outdoor levels of humidty changes, I suspect installing the wood when the humdity is about 1/2 way between the minimum and maximum of the year and definitely paint it completely if not seal it with a wood sealent.

Wood Drying (Mark).

Wood movement references
FNR 163 Published by Perdue University, Cooperative Extension Service, West Lafayette IN.
From page 3:
“In general, the amount of shrinking and swelling which takes place is directly proportional to moisture content changes in the wood. Wood shrinks and swells the greatest amount in the tangential direction, about half as much in the radial direction, and about 0.1% to 0.2% in the longitudinal direction. “
U.S. Department of Agriculture
Forest Service
Forest Products Laboratory, Madison, WS
U.S.D.A. Forest Service Research Note FPL-0203
From page 4:
“In the United States, the commonly accepted information on longitudinal shrinkage was summarized in 1931 by Koehler: “The longitudinal shrinkage of normal wood ranges from 0.1 to 0.3 percent.”
National Research Council Canada CBD-244.
From pages 1 & 2:
“Wood shrinks (or swells) not only tangentially and radially, but longitudinally as well. Tangential shrinkage (concentric to the growth rings) is approximately twice the radial shrinkage (perpendicular to the growth rings). Shrinkage values for individual specimens of the same species can vary considerably, so computed values based on averages may be somewhat misleading. The average tangential shrinkage of spruce from the fibre saturation level to the oven-dried state is 7 to 8%, while the average radial shrinkage is about 4%.1 The longitudinal shrinkage for most species over this moisture range, however, is only 0.1 to 0.2% for so-called "normal" specimens. This small value is usually ignored in design, sometimes with unfortunate consequences. Greater longitudinal shrinkage can occur if the wood is badly cross-grained or contains juvenile or compression wood. Juvenile wood comes from trees that grew rapidly during their early years. Compression wood, i.e. wood subjected to unusual compression stresses during its growth, usually results when trees grow on a slant. It also forms immediately below large branches, so that lumber with many knots may exhibit greater-than-normal longitudinal shrinkage.
Plywood has shrinkage characteristics similar to lumber in the longitudinal direction. This stability is due to the much higher modulus of elasticity of wood with the grain than across the grain. Alternating the direction of the grain in adjacent plies, therefore, stabilizes the plywood in both directions.
Waferboard benefits from a similar stabilizing effect because the individual wafers are randomly organized.  If waferboard is soaked, however, the resulting increase in thickness can so weaken its internal bonds that it exhibits greater movement than would normal lumber.”
From page 4:
Wood Truss Uplift
“An increasingly common effect of wood shrinkage is the upward bowing of wood trusses in winter This causes cracks between the partitions and the ceiling of up to 20 mm in severe cases. Wood truss uplift is primarily caused by the differential longitudinal movement of the upper and lower chord members.
Air in a well-ventilated attic space contains approximately the same amount of moisture as the outside air.  In winter the relative humidity of the outside air is fairly high; consequently, the top chords and web members will absorb moisture until equilibrium is reached with the surrounding air. The higher moisture content causes the top chords to lengthen.
The lower chords, however experience a different phenomenon. Since in modern houses they are often covered with up to 300 mm of insulation, their average temperature in winter is closer to the indoor temperature. This causes the air spaces in the insulation adjacent to the wood to have a much lower relative humidity than the air adjacent to the top chords.  As a result, the air spaces adjacent to the bottom chords absorb moisture from the wood until an equilibrium moisture level is reached. The moisture content in the lower chords may decrease to less than 10% during the coldest winter months, and cause the chords to shorten.  As the lower chords shrink and the top chords expand, the peaks of the trusses are forced upward. This forces web members attached near the peaks to pull the lower chords upward, which, in turn, causes cracks between the ceiling and the partitions. If the chord members contain compression or juvenile wood, the amount of movement can be significantly increased.”
National Research Council Canada   CBD-85
From page 1:
“For Canadian woods the range of radial shrinkage is from 1.7 to 6.7 per cent, that of tangential shrinkage from 3.7 to 10 per cent.  The shrinkage along the length of the grain is not shown, but it is normally very much smaller, being from 0.1 to 0.3 per cent in total, except for some abnormal conditions that can develop. “
U.S. Department of Agriculture, Forest Service
Forest Products Laboratory
Madison, WIS.
U.S. Forest Service Research Note FPL-073
From page 3:
“Veneer, plywood, and flakeboard specimens were subjected to various humidity conditions. Physical and elastic properties of the veneer were determined and the influence of these properties on the movement of plywood and flakeboards fabricated of like material was evaluated, The linear movement of the plywood and flakeboards was closely related to the longitudinal-to-grain movement of the veneer.”

Updated 10/14/14