I saw this pointed out in an episode reaction thread, but I’m pretty sure “Tomorrow and Tomorrow and Tomorrow” marks the first time the Federation has been explicitly, on-screen referred to as “socialist”. Fans, of course, have been calling the Federation socialist/communist for a long time. I think it started kind of crystalizing in 90’s Trek, particularly on DS9 and with Picard’s little speech about not using money in First Contact.
It’s kind of a throwaway moment in the episode, but it feels big to actually use the word? I suppose modern audiences are much more comfortable with the word than they would have been in the 90’s.
Trains don’t cover literally every single use case of a car, better keep expanding those highways!