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.
Green apples are notably sour, and apples are associated with Adam and Eve, the cost of desire etc. The speaker has eaten a whole bag, not just a bite. 
 The literal image is of the physical sense of fullness and carrying weight. It is clearly a negative feeling– guilt, physical discomfort of overeating equates to carrying weight of new life.
Metaphors