So it turns out all of the rails in my old blueprint book are now labeled as legacy rails. They are semi compatible with the new rail system, but not completely. To the credit of the devs, they work together so seamlessly that I didn't notice until I hit the 50 hour mark for my current save.

But that's the issue, at least for me. About half of my rail network is legacy rails, the rest the new stuff. And worse, the grid doesn't perfectly align between them, so creating new blueprints that align with the old rails doesn't really work.

From what I can tell, it doesn't really effect anything. But if any of you are about to start a new save and use an old blueprint book, it's probably worth updating it all.