Poets use metaphor to explore the ideas, forces and powers that lay behind our rational thought and our rational conception of the world. Ideas as living, thriving entities reveal themselves through the juxtaposition of two or more familiar ideas.
In "Metaphors", Plath superimposes the idea of her "I" or ego with ideas of objects such as riddles, elephants, houses, melons, bread, money, and cows. The "I" also identifies itself with a verbs such as strolling, eating, and boarding. Since Plath may have thought she was pregnant when she composed this poem, the idea of pregnancy(9 letters, 9 months)hovers behind the poem as a metaphor for human creation and becoming. Metaphors are embedded in metaphors, the 9 lines contain metaphors that describe pregnancy, but that whole system of metaphors is a metaphor for the essential spiritual being that manifested as "Sylvia Plath". For example:
The sensual pleasure of the food with its resulting sickness from overindulgence and the reference to the human "fall" through the apple image points points to both pregnancy and to her conscious or unconscious decision to offer herself as a vessel of creativity. Her decision to create is irrevocable and may be fraught with peril. She later wrote a poem, "Getting There", that expanded on the train imagery of the last line.
Google explains all. I think.