I'm pretty sure that bit at the end implied that another version of themselves was left behind on the train and died some sort of terrifying, horrible death.
So like... that conversation that Kozakura heard. That was definitely their voices from an alternate timeline or something. Like if they hadn't been able to get away from the monkeys or something.
I know that her power fading was a great thematic choice, but also it's a shame they can't have raunchy sex in public while time is stopped. That would've been extremely based.