Why does Curie dislike it when I complete railroad quests? You'd think being a synth she'd like that.... She always talks about wanting to join the brotherhood, but that would never happen.... so what faction would be a good end-game choice for her as a companion?

I am also interested in this. Which factions does Curie like?

I'm surprised that she dislikes the Railroad as the OP states.

If you want to unlock her friendship/romance quest do Cambridge Polymer Labs quest with her and make the Power Armor. She'll talk to you right after that.

And I get why Curie likes the BoS more than the RR. Her personality was created pre-war. The BoS is all about not letting what happened in 2077 happen again.

I actually got her love/romance dialogue just after i finished off BoS with the Institute

So, Curie is cool with the Institute?

Like someone above said - she is always going on about the BoS, but that isn't going to work for her once she "evolves", so the Institute is a good choice for her?