the standards for originality changed substantially twice in the timeframe we are talking about (recently, in 2023, the Supreme Court made it more strict, reversing a ruling from the 90s)

but still, what counts as sufficiently transformative is a very complicated question, especially when it comes to this kind of thematically identical but still creatively innovative derivative works

I remember how many years artists spent on the NewGRF packs, to make them high definition, what to add, what to keep, what to reimagine, and how! and of course the same goes for the game mechanics, the UI, the ... pathfinding, the economics (prices for freight, but also now there's a supply and demand system too, right?)

but the original behavior is still kept, so that's more clearly derivative (a copy), and in this case the new additions might be a new original work on their own, but still for OpenTTD to be able to be distributed without infringement of the original TTD it either needs to license that work for redistribution or remove the infringing parts.

(and of course it's a layman's analysis, so ...)