Solving Riddles

Although I don’t think I understand a lot of  poetry, I do enjoy it, and have a small collection of poetry books on my shelf. Poems are like riddles, and I do get a sense of accomplishment when I figure them out.
The other day I enjoyed a discussion on a poem called “Metaphors” by Sylvia Plath.

Metaphors

I'm a riddle in nine syllables.
An elephant, a ponderous house,
A melon strolling on two tendrils.
O red fruit, ivory, fine timbers!
This loaf's big with its yeasty rising.
Money's new-minted in this fat purse.
I'm a means, a stage, a cow in calf.
I've eaten a bag of green apples,
Boarded the train there's no getting off.
This poem was discussed on Midday Connection, one of my favorite podcasts. An English professor from Moody Bible Institute is a regular guest on the show, as the Book Club host,  but this time she was helping listeners interpret a couple of poems. Interestingly, she mentioned that she rarely assigns poetry writing. Her goal is to encourage students to enjoy poetry, seeing the deeper meanings hidden in the descriptive language. And she does that very well.
Can you guess what this poem is about?