That's true, at least around here [Germany]. Plus the nasty stuff might leak into the environment over the decades, and it's a pain to dispose of.

IIUC, they replace them with plastics since the plastic is seemingly more ecologically friendly and easier to recycle.

Mind concrete sleepers are what's used these days. You'll find wood only in shunting or cargo yards. (Or museums. Or the US - see linked commen by LeonM).

Source: I randomly met someone involved with that project. A proper train enthusiast can probably elaborate here, but I think I remember the core idea correctly. Also this obviously doesn't necessarily hold globally, though I can imagine many track operators face similar challenges.

Edit: While I wrote the above, LeonM wrote a nice reply on a sibling comment - https://news.ycombinator.com/item?id=44028161

I think pressure treated implies chemical impregnation--the pressure is used to diffuse the preservative through the wood.

Your fence probably has chemicals like that too.

then the carbon trapped in that wood is staying there for much longer than it would otherwise