I think Grace will get off the train when she's in her early/mid-20s, after all the kids in the Apex have gotten their exit, then she spends a bit doing some self reflection and traveling on her own (and probably grieving) for a while before she's ready to leave. She considers trying to look for Hazel and Amelia, but after going through car after car after car, she comes to decide that Hazel is likely better off without her*, and decides to leave her alone. (I'd really want Grace and Hazel to see each other again, but idk I can't really see that happening in canon)
I could either see her:
I'd want to explore either of those options in the future, maybe in a drabble or a full blown fic
* (this is subjective, as I'm not particularly keen on Hazel staying with Amelia given her attitude towards Hazel and kids in general. But on the other hand, Grace did end up hurting Hazel even if she was trying to do what's best for everyone. As much as I adore their relationship, I think Hazel deserves to be with someone who won't throw her under the bus to save face.)