I don’t get it but I think it’s supposed to be sad
It's historical drama from ~80 years ago. The women are insinuated to be lovers, the one in the train decided to marry in a traditional japanese fashion, joining husbands family and moving to their family house, and cutting/reducing her relations with her old life. The woman in black went to say goodbye, pretending to be all collected about this, but in reality being a total drama queen pissed off at her "friend". Or more likely she is super sad but ran out of tears and accepted it at this point.