Let us consider the poem as a whole before we move on to focus on the particular line that you have highlighted. This poem is used as a vehicle for Plath to express the mixed emotions she has at being pregnant. Almost every line contains some sort of cliche used to describe pregnant women, and we can see that through this technique Plath indicates the way she feels that being pregnant has made her lose her identity and now who she is is only consumed by the fact that she is pregnant. However line eight is a rather intriguing line:
I've eaten a bag of green apples
This is not a metaphor for being pregnant, however, perhaps it explains the bitter feeling she has in her stomach that you get from eating too many green apples. This is yet one more line that shows the speaker's profound ambiguity concerning being pregnant, pointing towards a sick feeling inside of her as she has now boarded "this train" and cannot get off.