# Thermal Expansion of Track and Roadbed



## rhyman (Apr 19, 2009)

I have had several off-line questions about my use of expansion joints. I thought this post might help take some of the mystery out of the whole thermal expansion issue. Listed below are the linear thermal expansion coefficients for some common track and roadbed materials below. This number is the distance in 10-6 inches that one linear inch of the material will expand with a one degree (F) increase in temperature.
Aluminum rail 12.3Brass rail 10.4Stainless steel rail 9.6Concrete 8.0Steel rebar 7.3Brick masonry 3.1Wood (fir) 2.1Wood (pine) 2.8PVC 28[/list] 

For example, a 36-inch long piece of brass rail raised in temperature by 25°F will expand 0.00936 inches.(36 * 10.4 * 10-6 * 25) Similarly, a 36-inch long section of concrete roadbed raised by the same 25°F will expand 0.0072 inches. (36 * 8.0 * 10-6 * 25) In other words, the rail will expand 0.00216 inches more than the roadbed. I realize that two thousandths of an inch doesn’t seem like much, but let’s look at an example with a longer section of track and use a more reasonable temperature variation that might occur between winter and summer. 
In this example I will use a 100-foot long section of brass track on the same concrete roadbed. This assumes that the ends of the individual rail sections are rigidly joined with some type of rail clamps and that the concrete roadbed is one continuous monolithic pour. Don’t worry whether or not the concrete has steel rebar. The thermal expansion coefficients for concrete and rebar are very close. I will perform the calculations based on a 70 degree temperature change (from 32°F to 102°F). The brass rail expands 0.8736 inches (1200 * 10.4 * 10-6 * 70), while the concrete roadbed only expands 0.672 inches (1200 * 8.0 * 10-6 * 70). The difference between the two expansions (0.2016 inches) could result in a kink somewhere in the length of the rail. 

If you use a combination of aluminum rail and PVC roadbed like I do, then the difference is even more severe. That is why I place a 1/16” expansion gap between every six-foot length of rail and between every eight-foot section of roadbed.

Hope this helps folks understand why thermal expansion can cause problems on our outdoor layouts.


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## Greg Elmassian (Jan 3, 2008)

Another comment, it does not take very much expansion at all to make a kink or a gauge change in the track. Just a few thousandths will make a difference. It's surprising how little expansion can cause a problem. 

Regards, Greg


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## sandbarn (Feb 13, 2010)

Bob, 
Is the PVC value suppost to be 28? Not maybe 2.8 or .28? Just seems like a lot for what I use for water pipe. I don't know which is correct, just asking..


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## rhyman (Apr 19, 2009)

LLoyd,
Unfortunately, the thermal expansion coefficient for PVC is indeed 28. A list of many thermal coefficients is contained in the Engineering Toolbox. These handy lists, tables, and formulas are a valuable resource for designing just about anything. Plus, they are free! Click here: link to engineering toolbox


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## armorsmith (Jun 1, 2008)

Bob, 

I am so glad you posted this thread. I have been knocking my head against the wall with some fellow modelers in my club over this very issue. 

On a personal note, I borrowed a friends Infra-red thermometer in mid September here in the panhandle of Florida and got readings on as high as 140 degree rail temps. I believe these could be as much as 20 degrees higher in the height of summer. We do get down in the 20's occasionally and I have done the math - ouch that's a bunch of expansion over the entire range. 

Bob C.


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## RimfireJim (Mar 25, 2009)

hat is why I place a 1/16” expansion gap between every six-foot length of rail and between every eight-foot section of roadbed. 
Creating an expansion gap without factoring in the temperature of the components at the time you are creating the gap is a bit of a crap shoot. Imagine if you were laying track on the hottest day of the year (OK, not too likely or bright, but bear with me). In the brass/concrete example, the gap should be at a minimum under those conditions. If you make it 1/16" by default, then the gap will be 1/16" bigger than necessary at the cold end of the spectrum. If you are using aluminum and PVC, the situation is reversed. Your rule of thumb probably works because you lay your track during moderate temperatures, somewhere between the extremes. 

I see this same mantra in woodworking magazines when advising to allow for hygroscopic expansion - "leave a 1/16" gap around the edges of the panel to allow for expansion". I have yet to see them say to take into account the ambient conditions and/or moisture content of the wood at the time. If it is really low humidity, like the 10-20% it can get here during a Santa Ana condition, 1/16" isn't going to be enough on a big panel (like in a raised-panel door) when conditions change to 80-90% RH.


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## Dr Rivet (Jan 5, 2008)

Bob 

That is really an eye opener. Based on the length of my track and using the SS coeff as an approximation for my NS rail, During a one year cycle I have to account for 6.5 inches of expansion. No wonder my rails keep creeping around the layout. 

Thanks


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## Semper Vaporo (Jan 2, 2008)

Posted By RimfireJim on 02 Dec 2010 07:46 PM 
hat is why I place a 1/16” expansion gap between every six-foot length of rail and between every eight-foot section of roadbed.
Creating an expansion gap without factoring in the temperature of the components at the time you are creating the gap is a bit of a crap shoot. Imagine if you were laying track on the hottest day of the year (OK, not too likely or bright, but bear with me). In the brass/concrete example, the gap should be at a minimum under those conditions. If you make it 1/16" by default, then the gap will be 1/16" bigger than necessary at the cold end of the spectrum. If you are using aluminum and PVC, the situation is reversed. Your rule of thumb probably works because you lay your track during moderate temperatures, somewhere between the extremes. 

I see this same mantra in woodworking magazines when advising to allow for hygroscopic expansion - "leave a 1/16" gap around the edges of the panel to allow for expansion". I have yet to see them say to take into account the ambient conditions and/or moisture content of the wood at the time. If it is really low humidity, like the 10-20% it can get here during a Santa Ana condition, 1/16" isn't going to be enough on a big panel (like in a raised-panel door) when conditions change to 80-90% RH. 

The first track I had was all plastic. I layed about 40 ft of straights on some 1x6 boards laying on the ground to experiment with my new Aster Mike. I feared the wind blowing the track off and breaking the molded snap joints that held the track together, so I put ONE nail in each end of the 40-ft. A few days later I came home from work when it was really hot out. As I drove up the driveway I noticed the track was elevated in the center by about 2 to 3 ft, forming a neat "bell curve". After I parked the car, I walked over to it and just barely touched the center of the curve and it immediately fell over, breaking nearly half the plastic joints!









My next track was about 150-ft of code 215 Nickel silver (Llagas Creek) in a double-loop back config. Each loop was about 17-ft in diameter and the two were connected together with what was left of the 150-ft of track. The whole thing was elevated about 3 to 4-ft on a wood structure of 1x6 boards.

Again I feared wind blowing the track off and bending the rails, but figured if I nailed it all down it would still bend the rails due to expansion. So, I used nylon fishing line to loosely tie the track down, figuring the track could slip around a bit to allow for expansion and contraction. I passed the line under one rail, then between the rails I passed it over a one or two ties and then under the other rail and tied the ends together under the wood structure. Those one or two ties per 6-ft long section of track held down the whole section.

To joint the sections I used the curled spring-metal track jointers that grip the foot of the rail. I placed a dime or penny between the sections while assembling the track as expansion space. I did this while the outside temps were "pleasant" and most of the track was in the shade.

When I first started playing with my trains outside, I would put a boxcar on the rails and walk around the track pushing it ahead of me watching for twigs and other problems on the track... never had a problem.


When it got really hot out and the sun was shining hard on the track, it heated up and expanded and eventually all the joints closed up, just as planned.

When the track cooled, instead of all the joints returning to the penny width gap between the ends of the rails, just the one joint with the weakest grip on the rail gave way and it opened the sum of all those penny gaps I had left in the original configuration... far enough for one rail to come out of the clip, creating a GAP in the track!

Unfortunately, about that time I was bored with the track-inspection routine, since I never really found anything bad. I got my Aster Mike all fired up and it got 1/2 the way around the EastEnd Loopback and derailed at that gap, falling 3-ft to the ground. Aster's are really nice engines, but they are not designed to FLY!

I removed almost all the nylon fishing line to free up the track to move where ever it wanted to and floated it all in ballast (loose "chicken grit"). I also went over all the joints, squeezing out all of the gaps that were left and giving each of the metal clips a good crimp with a pair of pliers to increase their grip on the rail.

Now, when it is hot the track expands sometimes to the point where the loops get so big they get close to the outside edge of the boards it is on, but when it is cold the track is close to the inside edge of the boards. Once or twice a year I may have to give the track a good jiggle to force it back to about the center of the boards so the whole loop does not creep off the edge in one compass direction.


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## kormsen (Oct 27, 2009)

PVC is a real mover! 

with our extreme temperatures here in the semi desert we use to give the PVC irrigation pipes a full percent of slack for contracting. 

the contracting/expanding business is - in my eyes - the strongest argument for sectional track. 
every one-foot section fixed with one nail and expansiongaps of about the thickness of two fingernails (at freezing temperature) compensates temperature differences of 100°F without kinking, raising or bending. 
(mind: that is using the slip-on or "fishplate" connectors, not clamps!) 
only the clicketyclack gets a little louder with falling temperatures. 
.


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## Totalwrecker (Feb 26, 2009)

The only negative issue I've had with contraction was the first winter, where a curve entered a tunnel and my 'long' Sierra caoches rubbed the portal along their middles. My 14" chain saw quickly remedied that! 

I use AC's SS track and use the small screws in the joiners or clamps where custom fits went in. 80% of my mainline floats in ballast and 20% is through 2 bridges and around a 90 degree curved trestle. The track also floats here as it isn't secured. I did add an extra stringer on the trestle so as the track expands and contracts it is always well supported. In 5 years I've never had a kink. I also use floating stringers on my 4' long truss bridge, they are secured to a few ties. 

My temps range from 20 to 120 degrees.... with some extreme ranges say from 32 to 80 in 24 hours.... last year I added a wye, all legs connected and a revesing loop and haven't seen any complications from complex track work..ie; more directions to move and some blocked by track config. After the tunnel experience, clearance tolerances were increased and no problems have occurred. Survived another Global Enegy Added summer with no ill affects other than Insta town taking flight from a downward micro burst wind! 

As long as I can get back up, I'll keep running on the ground..free floating. 

I did toy with the ladder idea and elevating the line, but the selling point of G was working with the Earth instead of fitting earth to the RR as was the norm on my smaller scale RR's. Cuts and cribbed fills are my concessions to undulating terrain. 

I can 'see' where a narrow ladder can cause all sorts of problems as I watch my track move 1"-1 1/4" laterly on the trestle. Of course here is where the 10' rule helps, I've never run track over a trestle before, always spiked rail to closer spaced bridge ties, never a problem in a climate controlled home, but outdoors in the desert? I'm lucky, I know, to have the land that is accomodating to a railroad, but I also built it to be accomadating to Ma Nature. 

John 

John


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## RimfireJim (Mar 25, 2009)

When the track cooled, instead of all the joints returning to the penny width gap between the ends of the rails, just the one joint with the weakest grip on the rail gave way and it opened the sum of all those penny gaps I had left in the original configuration... far enough for one rail to come out of the clip, creating a GAP in the track! 
Great anecdote, SV. Illustrates the difference that can occur between theory and reality. 

A lot of good info in this thread. Really making me re-think my plans to use PVC subroadbed for aluminum track in conditions much the same as Totalwrecker's. My line will be in a raised bed, and I was drawn to PVC as a construction-friendly means of avoiding problems with soil settling. That doesn't looks so good now.


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## Ron Hill (Sep 25, 2008)

Bob, I can attest to the fact that brass rail expands and contracts due to temperature. The first track on the layout had a large curve on the south end that was constantly moving. I would level it one day, and it would be out of level the next causing my Aristo Craft Dash-9 to derail. It was a constant pain in the neck because it was on the south end in full sun on hot Mississippi days. I installed an expansion rail section on both sides of the curve and it really made a huge difference. The track still moved, but not near as much as before. There is not anything that can be done to natural thermal expansion in metal, but thank goodness there is a way to somewhat offset it. 
Ron Hill


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## armorsmith (Jun 1, 2008)

Bob H. 

Can I assume that the coefficient you list for 'Pine' would cover Pressure Treated Southern Yellow ? My spline roadbed will be made from PT and I will be factoring in all the thermo-goddammits during the design phase. 

Thanks, 

Bob C.


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## Dan Pantages (Jan 2, 2008)

"Creating an expansion gap without factoring in the temperature of the components at the time you are creating the gap is a bit of a crap shoot." 

The British Columbia Society of Model Engineers has a couple of sets of gauges marked in degrees. They use 20 foot lengths of steel "band" for rail, set in slotted ties. Whatever the temperature is when they are laying track is the "gap" gauge they use. With a little work you could do the same with Gauge 1 track. This is what I did.


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## jjwtrainman (Mar 11, 2011)

That is why you should build in the height of summer and use rail clamps. This helps because that is as large as the track will get and the rail clamps will allow for shrinkage. 
plus, if a may, The SR-71 Blackbird (an airplane used for military operations) has incredible heat applied to it when in full flight and only results in a 2'' increase in length over some 120ft. This means that expansion is an issue, but for most small garden railroads, it is best to have 1 or 2 expansion joints next to the outside of curves. 
--JJWtrainman


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