I think the speaker is an expectant mother. There are many proofs showing that she is pregnant. For example, the poem is composed of nine sentences. And each sentence has nine syllables. It means that a baby needs to stay in mom's womb for nine months. The second sentence says," An elephant, a ponderous house". The pregnant women will become fatter and fatter, also their figure will be bigger than before. I think the speaker compared a pregnant mother to a weighty and huge elephant. And the "ponderous house" means that her womb or belly becomes heavier, because the new life will grow up in this cozy place. The third sentence " A melon strolling on two tendrils", means that her melon-liked belly is swaying right on the top of her two feet. The forth sentence says that" O red, ivory, fine timbers!". It shows that how she loves her child. She cherishes the baby as valuable as ivory, thinks her kid as cute as red fruit and regards the child as perfect as fine timbers. "This loaf's big with its yeasty rising" shows that her tummy is bigger and bigger as the fermenting bread. "Money's new minted" obviously compared the new-born baby. In this poem, the house, the melon and the fat purse, even the loaf of bread, all refers to women's tummy. We humans' life are all given from females, so we can say women are kind of means or stage to produce or transfer a new production. In the last two sentences, it seems that the speaker regretted being pregnant. She hated to be pregnant. "I've eaten a bag of green apples", the green apple tastes not very sweet. The taste of sore and astringent is like morning sickness. And the kind of nauseating feeling makes her disgusted, so she wants to get rid of it and yearns for returning also. However, it's impossible for her to return, just like she doesn't have second chance to get off the train when it has boarded.