I think the registered trademark is Transport Tycoon not TTD. The public may have called it TTD, but I'm not sure they used it in trade (IANAL, not legal advice). And I don't think the OpenTTD devs call it "Open Transport Tycoon Deluxe" anywhere. So the trademark case against the OpenTTD devs is likely pretty weak.

I personally think it is the copyright that is the most uncertain. Firstly, there are probably quite a few venues around the world where Atari might be able to take this up, and quite a diversity of precedent between them. Historically Atari litigated in the US - in 1981 they lost https://en.wikipedia.org/wiki/Atari_v._Amusement_World on a case someone infringed by copying their look and feel without copying any assets. Other precedents in that jurisdiction have found it's not infringing if similarities are inherent to the subject matter of the game: https://en.wikipedia.org/wiki/Data_East_USA,_Inc._v._Epyx,_I.... but similarity of art style is copyrightable: https://en.wikipedia.org/wiki/Tetris_Holding,_LLC_v._Xio_Int....