If I'm understanding this story correctly, Rie already boarded the train before the story and probably met with her other self. The other self may have done something to the original Rie and came back to the original Rie's world ( hence the cut hair ). The mirror Rie probably took Keiko the the mirror world just to show her how different it was. At the end, Rie holds the R keychain which was most likely taken from the original Rie.
