Jacobpauls1:
I wasn't planning on using small profile furring. At Lowes they have 1 x 4 furring product.
Thanks...
If you're really intent on using it, why not build one module and see how it turns out. Like I said, my cheap 1x3 frame held up well enough for my Christmas layout even though they were not all straight and there was some warping. There was also some breakage where there were large knots, but some of that might have been from the flexing when I moved it from the dining room to the garage and back a few times.
I don't know what your timeframe for construction is, but another option might be to buy the straightest piece you can find and let it sit for awhile to see what happens to it over time.
The bottom line is that no one is going to suggest using furring grade lumber, even if it's 1x4s. Does that mean it won't work? No, it just means that experience says chances are higher that it won't work well. There's a reason why it's cheaper, so it's up to you to figure out if you're willing to deal with the potential problems.
BTW, how big of a layout are you planning to build? Is it going to be open grid or tabletop? If tabletop, are you planning to use 1/2" or 3/4" plywood or something else? If plywood, what grade? The cheapest they have or the 7-ply 3/4" birch everyone recommends? Will it be a permanent layout for you or a somewhat temporary layout for the kids? Is the space dry or subject to swings in humidity? I'm having a hard time too looking at $50 sheets of plywood and wondering if it's really necessary to spend that kind of money just to play with trains. I figure my layout is going to cost around $400 in plywood if I rip them into 3 1/2" strips for framing. Each 8' strip will run about $3.50. Is if really worth $1.50 per strip or about $150 overall if I'm willing to shell out $300-$500 for a single engine?