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.
When you get on a train the doors are sealed shut until the destination has been reached. The speaker is too far along in her pregnancy to turn back or terminate. She could also very well be on the way to the hospital to deliver… Either way her babies are coming whether she likes it or not… 
 Once the baby is born, as well, she is pledged to its love and care, and the motherly instinct ensures there’s “no getting off” that responsibility.
Metaphors