A materials question

ABS is about .0001 and brass .00003 per ° (brass is more stable)

Have you considered basswood instead?   Relatively stable and prototypical.   

Mitch

No, I hadn't, but I'm going to look into what's available in wood. I was concerned about how fragile and susceptible to humidity such a narrow and thin strip of wood would be, especially since it would be suspended between the supports and not strengthened in any other way. But others have also suggested wood. Thanks.

Be sure the wood is relatively dense.  For example, popsicle sticks and the like spend their time stuck in frozen water without warping, so think along those lines...   

Also, think in terms of angled expansion joints, such as prototype railroads use on welded rail. 

Kappler Lumber.    I used their products for a turntable bridge.

Extended ties that matched the Atlas ties.

Scale 2" X 12" material used for the walking deck.

@M. Mitchell Marmel posted:

Be sure the wood is relatively dense.  For example, popsicle sticks and the like spend their time stuck in frozen water without warping, so think along those lines...   

Look at coffee stirrer sticks -- not basswood and usually birch or something similar in hardwood that will be very stable

The crafts project, summer vaca., built a diorama from moss,  pine cones, and popsicle sticks.  Grand daughters  3, 5, and 8, Ashlynn, Heather, and Hanna.  I helped with the cabin/popsicle sticks, used carpenters glue/ white glue.  Very stable, little or no bend/warp.  

@mwb posted:

Look at coffee stirrer sticks -- not basswood and usually birch or something similar in hardwood that will be very stable

Even better; they're already cut close to scale.  Excellent suggestion!    

Mitch

After considering brass strip, plywood and coffee stirrers, I settled on basswood. We'll see how it performs in next summer's warm weather.

Joe,

Love the 3rd rail. Did you use HO code 100 flex track? I was thinking I would use thin strips of balsa wood for the covers.

Havent done a mock- up yet.

Glen

Code 125 rail by Micro Engineering from www.fast-tracks.net

The basswood was 1/32" x 3/16" which I estimated scales to 3" thick x 9" wide. Sounded like ballpark to me.

Didn't consider balsa. Good enough for model airplanes so maybe it could work for this, too.