I run the following in my "mod of mods" mod, wondered if you might add this or something similar:

-- add glass to rail signals
if mods["crushing-industry"] then
    recipe = data.raw["recipe"]["rail-signal"]
    if recipe then
        fdsl.modify_ingredient(recipe.name, "iron-plate", { amount = 4 })
        fdsl.add_ingredient(recipe.name, { type = "item", name = "glass", amount = 1 })
        if mods["quality"] then recycling.generate_recycling_recipe(recipe) end
    end

    recipe = data.raw["recipe"]["rail-chain-signal"]
    if recipe then
        fdsl.modify_ingredient(recipe.name, "iron-plate", { amount = 4 })
        fdsl.add_ingredient(recipe.name, { type = "item", name = "glass", amount = 1 })
        if mods["quality"] then recycling.generate_recycling_recipe(recipe) end
    end
end

Hiya! Yeah that's a great idea, I'll add this in the next update, thanks for the suggestion.

P.S: The FDSL functions auto-check if the recipe exists, so you can just specify "rail-chain-signal" as the recipe name and it'll no-op if the recipe doesn't exist. If the FDS_ASSERT is defined it'll assert then, but it's otherwise not a fatal error.

I plan on updating FDSL to 1.0 in a few weeks with a suite of unit tests and documentation so I can ensure they're stable and have proper documentation for other modders to reference. Alongside this, I may add two modes for either asserting or simply logging a warning so you can keep developing and clean up those specific cases if it's critical.

Rail signals now have 1 glass in 0.3.6! I decided to omit the iron plate cost since I don't want to override balance changes done by other mods and it's a minor difference.

The exact recipe did not matter, my hope was just to get glass into the mix.
Thank you!

While you're here, I'm curious: what do you think about the concrete mix recipe? It still takes (crushed) iron ore, but I think in the context of mixing into a liquid it felt more appropriate. I didn't add any iron sticks to the concrete item recipe to keep it simpler. I've considered doing so to make it easier to craft quality concrete, but there technically is another way to do so (without recycling) which I think is a fun little easter egg.

Speaking as one that is not a great fan of iron-ore in the vanilla concrete recipe, I will likely change it. But maybe not :D
I have not yet made concrete, but IR3 or K2 used liquid concrete, and I liked it. No doubt the realism played a part, but it felt right.
From a game-play perspective I like it.

I do wonder about the impact on recycling and the available iron. I thought concrete was a major source, but I am guessing that is not a deal breaker.

Ah speaking of recycling, I had intended to leave the concrete recycling recipe unaffected so you can acquire stone bricks from the recipe (since that's necessary for progression), but I guess I broke that at some point, will fix with the next update.

But RE iron, concrete recycling is actually a very insignificant source, since it's 1 iron ore per 10 concrete, so the ratio is very low. It's more for stone bricks