Granny Ellen Weatherall is on her deathbed, being tended to by the young village doctor and one of her daughters, Cornelia. She thinks back on her life and everything she has accomplished—much of it on her own after the death of her husband. She also thinks about how she was jilted at the altar by the first man she was supposed to marry, and she considers how she would like to let him know, somehow, that her life was a good one despite his heartless and very public rejection of her.
Granny Weatherall considers how nice a house she kept when she was younger, how neat and tidy it was then, as well as how much work she could get done. She considers how her children still come to her for advice about their own families or businesses; she still feels vital and useful. She thinks to herself that "She wasn't too old yet for Lydia to be driving eighty miles for advice when one of the children jumped the track."
This phrase is a metaphor that means to go off the expected path and to go in some unexpected direction, like a train that gets derailed. In this context, then, this phrase likely refers to Lydia's children, Ellen's grandkids, doing something crazy and unexpected, perhaps acting out and rebelling or something similar.